- Off-cut
-
A fragment of metal cut from the main body of an
artefact during manufacturing. Often fragments of sheet, rod or bar
with visible cut marks. This type of material may be important evidence
for the raw metal used, the type(s) of artefact produced, and/or the
manufacturing process. As well being preserved in non-ferrous metal
working debris, these marks can often be seen in the small fragments
of iron associated with iron smithing sites.
- Open Hearth Furnace
- A furnace of a reverberatory type fired
by producer gas and used air preheated using a pair of regenerative
heaters.
The use of open hearth furnaces was first patented for the melting of glass and metals
by Frederick Siemens in 1856, based on the principles set out in Stirling`s patent of 1812,
and Nielson's use of hot blast in the coke fire blast furnace.
The 1856 patent outlined the use of two sets of checkerwork firebrick heat exchangers through
which the air flowing into, or out of, the furnace passed. One set was heated by the
waste gas from the furnace, while the other (hot from a previous heating phase) was used to heat the
air going into the furnace. Periodically, the control valves were changed so that the air
flow thorugh the furnace and heat exchangers was reversed, the checkerwork that had been
cooled by the incoming air now being heated by the exhaust gas, and vice versa.
Initially the furnaces were fuelled by coke, or other solid fuel, but the ash carried
over by the air eventually blocked the checkerwork. This problem was eliminated if producer
gas was used as the fuel instead of a solid fuel. A further patent in 1861 by both William
and Frederick improved the furnace temperature by having a separate pair of checkerworks
to preheat the gas as well as the air. Such furnaces were capable of reaching temperatures
as high as 1600°C, and thus steel could be melted in them.
- Open Hearth Steelmaking
- In general, the term covers all methods of making steel using the Siemens Open Hearth.
Following the considerable savings in fuel costs by application of the Siemens regenerative
heating method in glassmaking, steelmakers tried to apply the same method to their industry.
Initially, open hearths were simply used as more efficient melting furnaces
for the production of crucible steel.
In 1864, as a result of their work at Sireuil, Pierre & Emile Martin issued
a British patent, through R.A. Brooman, on the melting of cast iron (pig iron) and puddled
iron or steel in a Siemens open hearth. This patent described a semi-continuous
method with an initial melt of cast iron, to which additional scrap steel and puddled iron
or cast iron was added as required to obtain the required composition. In following the year,
another patent improving on the method was issued. This outlined a batch process consisting of a
single melt that resulted in the steel maker having better control of the composition of the
steel. This
Siemens-Martin method of steelmaking was widely used through Europe for the next
15 years. Seimens described the Siemens-Martin process as the
pig and scrap process.
In Britain, from 1865 to 1869 Siemens experimented and developed a method of steelmaking
using the open hearth. By 1869 this had progressed to the extend that Siemens was making
75 tons of steel a week at his Landoe works, near Swansea. The method, which Siemens described
as the
pig and ore method, involved melting cast iron, then adding
iron oxide in the form of iron ore, together with some lime and other fluxes. The iron oxide acted
as a source of oxygen to decarburize the cast iron. Then as a final step before casting, -spiegeleisen
was added to the molten metal it was poured into ladle to be taken to the foundry.
In Britain, this was method was known as the "Siemens Open Hearth Process" (or the
Acid Open Hearth Process). For further information on this topic see
Barraclough 1990.
Related Terms - Basic Steelmaking,
Bessemer Steelmaking, Puddling,
Refining (cast iron), Steelmaking
-
- Open mould
-
A form of mould for flat or long object of relatively simple shape in which
the top is left open to the air. Examples of the type of object that were
cast in open moulds are Bronze Age flat axes, ingot moulds of all periods,
cast-iron pigs and fire-backs and probably Roman mirrors.
-
Order Hardening
-
A process which hardening of the metal occurs as result of an order-disorder reaction.
It is not widely used, but may be important in some noble metals, where the effect can be
used to advantage, but may also be detrimental if the metal needs to retain its flexibility.
-
Ordering
-
In binary alloys there is a preference for dissimilar atoms to surround
each other. Consequently certain alloys, when slowly cooled below a critical ordering temperature,
may undergo a transformation in which the atoms organize themselves into regular arrays of
alternating atoms over large regions (also call superlattice).
In the gold-copper alloy system there are three ordered alloys corresponding to the
compositions Au3Cu, AuCu, and AuCu3. The ordered phases are, in general,
harder than the normal or equivalent dis-ordered alloy. Therefore, quenching gold alloys of
these compositions from above the critical ordering temperature will keep these alloys in the
soft state, rather than harden them. However, the formation of ordered structures over time may
lead to embrittlement.
Ordered alloys differ from most intermetallic phases, in that they have a greater composition
range and that range tends to increase with decreasing temperature. This gives their phase
range in the equilibrium phase diagram a characteristic inverted U-shape.
-
- Ore hearth
-
-
A bellows-blown low open-fronted hearth used for lead smelting, in a water-powered smeltmill. Dressed ore (galena,
lead sulphide) was mixed with fuel in the hearth, and raked out intermittently onto a sloping 'work-stone' in front of the
hearth to partially oxidise the ore; the lead produced in the hearth ran out though channel across the workstone into an
iron pot, from which it was ladled into pig moulds. The chemistry was complex, but can be summarised as a
double-decomposition between oxidised ore (lead oxide and sulphate) and unoxidised sulphide, producing metallic lead
and sulphur dioxide vapour. The 'grey slag' produced was often resmelted in a separate
slag hearth. The ore hearth was developed in the mid 16th century; it was partially replaced by the
reverberatory furnace in the early 18th century, but
remained in use in some areas till till the late 19th century -later 19th century examples developed into enclosed shaft furnaces,
blown by pipes from blowing machines. The fuel was initially charcoal, but by the 18th century a mixture of peat and low-grade
coal was normally used.
-
- Orthoclase
-
The potassium rich end member of the alkali feldspars (KAlSi3O8)
group of minerals.
-
Overheating
-
Heating a metal or alloy to such a high temperature
that its properties are impaired.
- Oxidation (or Oxidization)
Chemical reaction in which a metal is converted to its oxide, or from one
oxidization state to a higher state (that is the equivalent of an increase in the number
of oxygen atoms for each metal atom. For example, in the reactions -
| 2Fe + O2 = 2FeO | Metallic Iron to Wüstite |
| 6FeO + O2 = 2Fe3O4 | Wüstite to Magnetite |
| 4Fe3O4 + 02 = 6Fe2O3 | Magnetite to Haematite |
iron is progressively oxidized from the metal to its highest oxide, haematite. Another
way of looking at these reactions is that oxidation occurs every time the cation
(the metallic ion) gains a unit of charge, e.g. Fe2+ goes to Fe3+,
or alternative that the cation loses an electron (the charge on an electron is negative
- e-)
e.g. Fe3 + e- = Fe2+
Related terms - Reduction, Valency
- PGE
- Platinum Group Elements;
- Paktong
- Corruption of Chinese 'Paitung' used for alloys containing
zinc. The other elements are usually copper and nickel; this alloy is synonymous
with a nickel silver or German silver.
- Panning
- A mineral washing process in which the lighter unwanted mineral is removed
from the wanted mineral in a shallow vessel (or pan). Panning was used as
a prospecting method, and for small-scale ore processing of gold deposits.
- Part mould
- A semi-permanent mould made of several pieces so that objects with recesses
can be cast without destroying the mould each time.
- Parkes' Process
- The Parkes' process was a cheaper method of extracting silver from lead. An
appropriate amount of metallic zinc was added to the molten impure lead. The zinc formed
an alloy with the silver, which crystallized out of the melt and floated to the surface
where it was skimmed off. The silver was then recovered by heating the alloy in a zinc
retort where the zinc evaporated leaving the silver behind,or by dissolving the zinc in acid. The
archaeology and archaeometallurgy of this process have not yet been investigated.
Related terms - Pattinson process,
Zinc smelting
- Parting
- Noble metals can be separated from base metals by liquidation of the metal
with lead, then cupellation of the lead,
however, this does not separate silver from gold. To remove silver from gold
a method of parting has to be used. This was by either acid
parting, or the salt cementation process, and
later by sulphur parting. For full discussion of
the history of parting see Rampage and Craddock
2000.
- Acid parting
-
Although weak organic acids could improve the colour of gold containing
copper, these would not remove silver, so that silver-rich gold would
remain a pale colour. The removal of silver required the use of strong
acids (hydrochloric and nitric in particular). Acid parting only became
possible when strong mineral acids became generally available. The distillation
is thought to have been first used during the eleventh or twelfth centuries
in Europe, although it is some evidence that they were used by the 10th
century in the Islamic world.
The method is not mentioned by Theophilus, but various 16th century European
metallurgical treatises and handbooks describe the method in some detail;
they suggest that it was not a cost effective alternative to salt parting.
However, it seems likely that it became more widely used by the end of the
16th century and during the 17th century.
- Antimony Parting
- See sulphur and sulphide parting.
- Salt Cementation Parting
-
Until the discovery of strong mineral acids salt cementation parting
was the main method used to refine gold. The literature would suggest
that there were a number of different variations on the process, but the
essentials were the use of common salt cement and the presence of alumino-silicate
ceramics preferably with an appreciable iron oxide content (in the form
of an earthenware container, and possibly the addition of crushed brick
or earthenware powder to the salt cement).
The impure gold was beaten into thin sheets, or was granulated to produce
fine grains, or was naturally in the form as an ore. The gold was interleaved
with layers of 'cement' formed of either salt or a mixture of slightly
moistened ground up alumino-silicate material (burnt clay - old pot or
tile) and salt (NaCl) in a sealed clay vessel. On heating the salt would
react with the alumino-silicates of the burnt clay together with the water
or urine, used to moisten the cement, to liberate volatile hydrogen chloride
or chlorine gas. This gas, in turn, reacts with the silver at the surface
of gold to form volatile silver chloride (AgCl), which was absorbed by
the alumino-silicate in the 'cement'.
The process had to be carried out at as high as a temperature possible
to speed the diffusion process. But the temperature was limited by either
the melting points of either the alloy, or of the active reagents if
no carrier medium was used. If the gold alloy melted, or the temperature
rose very close to its melting point, the foil would ball up. This would
reduce the surface area available for attack and increase the diffusion
distances, thus slowing the reaction. If no carrier material (brick
or pottery dust) was used, the gold would sink through a simple salt
cement if the temperature rose too close to the melting point of salt
(804 oC).
At the end of the process the gold would be melted into small ingots
that could be tested for purity and if found to be of low purity these
could be forged into foils so that the parting process could be repeated.
The spent cement would be smelted with lead, which would be cupelled
to recover the silver.
Various versions of the process have been described in the ancient
literature, with the addition of other salts such green vitriol (hydrated
ferrous sulphate), salt-petre (potassium nitrate) in addition to common
salt. However, the presence of large amounts of nitrates in the mixture
would result in the loss of gold as the combination of hydrochloric
and nitric acid will dissolve the gold as well as the silver.
Although widely used until the 18th century, the process was largely forgotten
and the distinctive debris from the process is rarely identified, as the
sherds do not have the vitrified internal surfaces typical of used crucibles.
However, parting vessels do have a distinctive purple tinge due to the
presence of silver chloride.
- Sulphur and sulphide parting
-
Finely divided impure gold could be treated with elemental sulphur.
All other metals would react with the sulphur to form sulphides. The process
had to be carried out at a low temperature as sulphur is easily evaporated.
The process could be carried out at higher temperatures if antimony sulphide
(Sb2S3)was used. Parting using antimony sulphide
was misleadingly known as the antimony process.
There is no evidence that either sulphur or sulphide parting was used
in antiquity, although sulphur was reacted with a mixture of copper and
silver to form
niello. The advantage
of sulphur parting was that it was much quicker than salt parting. It
was also believed to give much purer gold, but it was a more expensive
process. In addition, it was more difficult to recover the silver - the
sulphide had to be treated with iron filings.
The first definite European reference to the use of sulphur refining
is in Theophilus. It was used either when refined gold was required quickly,
or where gold of a higher purity than normal was required, i.e. to form
hollow-ware vessels.
Surprising, the first definite Islamic reference appears as late as the
14th century. Care has to taken with some translations, as some authors
seem to have translated terms for sulphate compounds as sulphides (Rampage
and Craddock p 36).
- Patina
- The surface layer formed by reaction with environment over time. However,
a patina may be formed artificially by chemical means for artistic effects,
or to fake an ancient appearance.
- Pattern
- Pattern Welding
- The technique of producing a pattern on a blade or other ferrous artefact
by the welding together of two or more types of iron or steel.
From the second century AD some Roman swords were made with a patterned
core fabricated by twisting and welding together several rods of low carbon
iron and phosphoritic iron. Steel cutting edges were then welded on to this
complex composite core. The resulting complete blade would be etched to
reveal the complex pattern of welds. It is likely that these blades were
regarded as high status objects, considering it would have taken about 70
hours to manufacture a sword (Anstee & Biek),
and that they were recorded as being given as gifts. Even so, it is likely
that they were mechanically inferior to a weapon made with a plain low-carbon
iron core as examination of the swords has revealed that the welds were
often poorly executed, with many slag inclusions, and even voids, at the
welds between the various decorative components (Tylecote
& Gilmour 1986, Gilmour and
Salter 1998).
In the early period the cutting edges do not appear to have been hardened,
but later the cutting edges were quenched. By the tenth century the number
of pattern-welded swords decreased, as swords forged from more uniform steel
replaced them. Pattern welded inserts were also used in seaxs (large single
edged knives) and spearheads
The technique of pattern welding reappeared much later as European craftsmen
tried to imitate the patterns seen in watered-steel (Damascus Steel) artefacts
made from a single piece of crucible steel. The technique was thus described
as damascening, in reference to the steel swords and artefacts sold through
Damascus. The technique is still used to this day to produce the intricate
patterns of very high quality shotguns.
In Asia two traditions of forming patterned blades arose by development
of the piling technique using dissimilar types of iron. In Japan, low and
high carbon steel components were welded together, then forged out, folded
and welded back on itself repeatedly, to form finely layered steel. On quenching
this produced alternate layers of pearlite and martensite, which were revealed
by etching and polishing. In Southeast Asia, a similar technique developed
in Malaysia and Bali but in this case the patterns were formed due to the
use of steel containing a little nickel (pamur). This could be from
meteoritic iron from the Prambana meteorite (now in the Kraton Gardens,
Surakarta), or smelted from nickel rich iron ore, possibly from Sulawesi,
in the Celebes Isles, (Craddock 1995,
273)
Related terms: Damascened, Damascus
Steel, Watered-Steel,
Wootz
- Pattinson Process
A 19th century development of the
cupellation process
for extracting silver from lead, depending on the principle that, when molten lead crystallises, the silver concentrates in
the remaining liquid while the crystals are silver-poor. The process was conducted in a row of cast-iron pans,
each holding 6-10 tons of molten lead, and heated by a coal fire. Lead was introduced to a pan at the centre of the row,
melted, and allowed to cool to crystallisation point. The crystals were scooped out with a perforated ladle, and passed to
the next pan in one direction, while the remaining liquid lead was ladled out to the next pan in the other direction. The process
was repeated in each pan, the crystals being passed in the opposite direction to the remaining liquid, so that the silver content
was progressively enriched in the liquid. The silver-rich lead from the end pan, containing up to 200-300 oz/ton, was then
treated by
cupellation.
- Pearlite
- A constituent structure that can be found in iron containing more than 0.15% carbon
consisting of alternating parallel lamellae of ferrite (Fe) and cementite
(Fe3C). It forms by the eutectoid
decomposition of austenite. It is a product of slow to moderate
rate cooling but may appear in conjunction with martensite
and troostite, or bainite
and troostite when the rate of quenching
is not high enough to form the fully quenched morphology, or in regions of
lower carbon content in inhomogeneous steels. Often in more slowly cooled,
or reheated steels the pearlitic structure has degenerated or spheroidised
to some extent.
Related term - Sorbite
- Peritectic
- An isothermal reaction in which one solid phase reacts with a liquid of
a different composition to form a new solid phase. The new phase may consume
all of the liquid to form a totally new solid. The formation of the beta phase
in the bronze system is an example.
- Peritectoid
- A peritectoid is the solid-state version of the peritectic reaction. It
is an isothermal process in which two solid phases react to form a new solid
phase. For example, a peritectoid reaction occurs in the copper-tin (bronze)
system, at 65% copper. On cooling from above about 580 ºC. the intermetallic
Cu3Sn, and solid solution gamma, react producing a new phase, Cu4Sn.
As such solid-state reactions are largely diffusion controlled they often
only occur to a limited extent, or may be completely suppressed by rapid cooling.
- Pewter
-
Ancient pewter is an alloy of tin and lead, much used in Roman times,
and from the Medieval to the 18th century. The poisonous nature of lead
has resulted in the replacement of lead with antimony, although antimony
is also inadvisable in high amounts for cooking utensils. Roman pewter appears
to have included three different binary tin lead alloys, the groups covering the
range 30 to 50%, 69% to 80% and greater than 90% tin. The first group would
seem to correspond to Pliny's 50-50 alloy, but the second has a higher tin
content (nearer to 3 tin to 1 lead) than Pliny's second alloy of two tin
to one lead. (
Pollard 1983 and
Hughes
1980)
Modern lead-free pewter is basically tin with variable amounts
of copper and/or antimony to harden the metal or alter the casting characteristics.
Such alloys may contain between 0.5-2.5% copper, and 0-10% antimony
- Phase
- A single component of the chemical assembly having a common physical and
crystallographic state.
For example - water, ice and steam are all different phases although they
have the same chemical formula. Silver in solid solution with gold is also
a single phase no matter what the composition, as there is complete solid
solution between the two metals. On the other hand, copper will only dissolve
to a limited extent in silver and vice versa. Thus most silver-copper alloys
consist of two distinct finely dispersed phases - copper with a very little
silver in solution, and silver with a little copper in solution.
- Phase diagram (equilibrium)
- A diagram with axes of temperature and composition describing the different
phases that will be present in a chemical system or alloy at equilibrium at
a given composition and temperature. A binary phase diagram consists of two
elements. A ternary system consists of three elements; these are often plotted
as a triangular diagram with temperature as contour lines.
Related Terms: Ternary diagram, TTT-curve
- Phosphoric iron
- Iron with sufficient phosphorus present to increase the hardness of the
metal. As well as increasing the hardness, phosphorus also increases the rate
of work hardening of the metal, and its brittleness. In 17th-18th century Britain, phosphoric
iron was referred to as 'cold-short', since it tended to crack during cold forging, but was preferred for nailmaking due
to its hardness. Phosphorus also increases the drawability of low-carbon wrought iron, phosphoric iron being used for
the wires of musical instruments (Goodway 1999 [JHMS, 33.2, 104-5]
The presence of phosphoritic iron may be revealed under the microscope by
excessive grain size (when little or no carbon is present) and/or ghost structures
when some carbon is present in the metal.
- Phosphorus ghosting
- In metallography of iron artefacts, regions in which differential relief
develops during polishing and etching due to phosphorus segregation in the
ferrite. These bands, which can cross the grain structure, are often best
observed by moving the sample slightly off focus.
- Piece-mould
- As part mould
-
- Pig
- In the earliest period of British cast iron production (16th century),
one of the products of the blast furnace was a large central ingot with small
ingots run off it. This was thought to look like a sow and sucking piglets.
The smaller ingots were termed pigs. Now the term has been widened to apply
to any similar shaped metal ingot destined to be broken up and remelted, although
most commonly applied to iron and lead.
- Pig boiling
- See Puddling
- Pig iron
- Cast iron ingots: Pigs of cast iron - or more generally broken cast iron
ready to be remelted. See above.
- Piled (steel)
- A method of improving the mechanical properties of iron and steel, in which
the metal is forged out, folded back on itself and the resulting 'pile' of
metal is welded together. This process may be repeated several times so that
the final metal consists of many fine layers.
Piling was used to homogenise steel produced by carburising strips of iron.
Because the carbon has to diffuse into the metal from the surface it is difficult
to produce homogeneous steel of any great thickness. If carried out successfully
the piling process reduces the final thickness of each original strip to a
fine lamination, while migration of carbon at welding heat further homogenises
the carbon content. Thus, although exactly the same methods are used as in
damascening (forging, folding and welding) the results and aims are different.
The aim was to produce a more uniform consistent structure, whereas, damascening
through the use of two different types of iron or steel deliberately produces
a decorative pattern. The aim of piling (although not always achieved) was
to produce a macro- and microscopically homogeneous steel, whereas, the aim of damascening
was to produce a steel that behaved in a uniform manner in the bulk, but was
patterned on the small scale. The production of
'shear steel', the highest-quality British product before the development
of crucible steel, was essentially a
large-scale version of piling, though given the thickness of the blister steel
bar raw material, micro-homogeneity could not be achieved (Barraclough
1984, 45-6 and 65-7).
- Additional comment
- Although, it is absolutely clear that some artefacts have been made
by this technique, it is likely that some that been identified as having
been made by this technique are, in fact, the result of either fortuitous
banding, or the side effect of other parts of the manufacturing process.
(In due course there will be another page added discussing
problems associated with the identification of piling and weld lines).
- Pinchbeck
- Gold coloured brass, invented by Christopher Pinchbeck, a watchmaker of
Fleet Street, London, in 1725. Probably made by melting copper and imported
spelter (zinc). Later the term was widely used for all gold coloured brasses,
and became a synonym for any cheap imitation. Thornton,
J. 2000, 311
- Placer deposits
- Placer deposits form by the release of dense minerals (or metals in case
of gold) by weathering of a primary ore body, and concentration by the action
of water. The most important placer deposits are of gold and tin (the latter
in the form of cassiterite), although locally magnetite may be concentrated
in black sands sufficiently for them to be used as an ore. Often, the deposits
are associated with the rivers flowing over a newly exposed landscape, rather
than the present river or beach systems. Thus, the alluvial 'stream' tin
deposits of Devon and Cornwall were buried several metres below the river
level. As well as alluvial deposits in river and stream valleys, placer
deposits include eluvial deposits weathered out from primary ores but only moved
downslope by weathering and soil creep processes.
- Plating
- The technique of applying a thin surface layer of one metal over another
for decorative, aesthetic, or anti-corrosive purposes. Plating may be applied
by a number of different means -
-
- Diffusion bonded plating
- Diffusion plating involves bringing the plating metal into intimate contact
with the metal to be coated at elevated temperatures, by application of
pressure or by mechanical working. The main use of this technique was for
the production of Sheffield Plate - silver-covered copper sheet. Diffusion
bonding was also used to produce the mokume multi-layer decorative metals
used in Japan.
- Dip or Hot Dip
- In this technique the objects to be plated were dipped in a hot bath of
the molten plating metal. This technique was used in the Iron Age to simultaneously
plate and braze iron horse fittings rings; examples of this technique have
been found at Gussage All Saints and Maiden Castle in Dorset, and Yarnton
in Oxfordshire.
Other examples of hot dipping to protect iron sheet from corrosion are tin
plating and zinc-coated galvanized
iron
- Electro-plating
- Electroplating uses an electric current to deposit the metal from solution
on to the object to be plated, which acts as the cathode of the cell. The
process was invented by Weiner Siemens in Germany, in 1842. His brother,
Karl Wilhelm, introduced the process to Britain in 1843, where he sold it
to James Elkington. This new electro-plating industry rendered the Sheffield
plate industry obsolete by the latter part of the 19th century
- Gilding
- A method of putting a thin layer of high carat
gold on a less expensive substrate. This includes a number of methods that
are given in more detail in the section on gilding - Depletion
gilding, Foil gilding, Leaf
gilding, Mercury or fire gilding.
- Vapour deposition
- A modern technique in which the object to be plated is put in a vacuum
chamber and the metal is thermally vaporized and deposited on the surface
as a thin film. Widely used to deposit aluminium on a wide variety of surfaces.
- Wiping
- In this technique the metal object to be coated was fluxed and heated.
The rod of the plating metal was rubbed over the areas to be plated. This
technique was typically used for the decorative application of low melting
point 'white' metal alloys.
- Platinum
-
Metallic element atomic number 78, atomic weight 195.09, mp 1772
oC,
specific gravity 21.45. First discovered by Western Science in South America by Ulloa
in 1735,
but had been used previously by the natives of Ecuador and Colombia. Finds
are known particularly from La Tolita dating to the last centuries B.C.
The metal is malleable and ductile, however, its high melting point, well
above the temperatures attainable in any medieval or earlier furnace, made
it very difficult to consolidate the native metal. Hence, the early South
American artefacts were of platinum
sintered
with gold.
In the 19th century AD an attempt was made to establish a Russian coinage
based on platinum coming from the Urals. In this case, the metal was consolidated
by sintering the metal at high temperature and pressure. This early attempt
at
powder metallurgy was soon abandoned due to technical
difficulties.
Native platinum is usually alloyed with some iron. Sometimes there is
sufficient iron present for the metal to be magnetic. Platinum was used
in early scientific instruments since its coefficient of expansion is very
similar to soda-lime glass.
- Platinum Group Elements
- The group of elements whose characteristics are similar to platinum, in
that they have high densities and melting points, and are unreactive. These
elements are iridium, osmium, palladium, rhodium, and ruthenium
- Platinum Group Element Inclusions
-
Platinum group element inclusions occur as small silver-grey or steely
coloured particles in alluvial gold. These were much harder than the surrounding
gold and caused problems to jewellery makers. Osmium-iridium inclusions
found in the California placer gold were used to make the hard-wearing Osmiroid
pen tips until quite recently, due to their high hardness.
It has recently been realised that the classical term, adamas, must have
been referring, in part, to PGE inclusions, as Pliny noted that -
'Adamas was the name given to the auri nodus (literally knot
of gold), found very occasionally in mines in association with gold and,
so it seemed, only forming in gold.'
from Rampage and Craddock 2000, 240,
referring to the interpretation of the text suggested independently by Ogden
1977 and Nicollini 1990.
- Powder Metallurgy
-
Powder metallurgy is a set of methods used, initially, to produce metal
components by sintering of pre-formed shaped powder compacts by the application
of heat and pressure.
The technique was first developed to deal with refractory and reactive
metals. More recently, the method has been applied to oxides and other ceramic
materials. Also, it has been use to try to make alloys with finely
divided and uniform microstructures which can niow be produced by any other
means.
- Poling
- A process designed to reduce the oxygen content of molten copper. This
is carried out by plunging a pole of wood into a bath of molten copper. The
distillation of volatiles in the wood results in the evolution of hydrogen
and other reducing gases. These, in turn, reduce any copper oxide present
in the molten metal.
- Polycrystalline
- Consisting of many individual crystalline grains. Most metals are polycrystalline
solids.
- Precipitation
- The process by which a minor phase or component comes out of solution due
to a chemical or physical change. For example -
'Iron carbo-nitrides may precipitate as small acicular
particles in ferrite subject to prolonged heating in air.' |
- Precipitate
- A small particle of a specific metallurgical phase that has formed out of
solution. For example -
'The aging of aluminium copper may harden the metal
due to the formation of small coherent CuAl3 precipitates.' |
- Prill
-
A small droplet of metal that has solidified from the melt. Prills are
often the result of splashes of metal lost during casting operations, or
are trapped in slag during a variety of metallurgical processes but in particular
smelting or assaying.
According to the Shorter Oxford Dictionary, the word originally was used
in Cornwall to describe the richer pieces of copper ore remaining after
the leaner ore had been removed by
cobbing.
Later it was used to describe the globule of metal obtained on assaying
the ore by cupellation. Hence, its present wider use to describe small metallic
droplets.
- Proeutectoid
- For an alloy with a composition away from the eutectoid composition a phase will precipitate
before the eutectoid component can be described as the proeutectoid phase or component.
In the case of a hypoeutectoid steel, ferrite will start to form on the austenite grain
boundaries at temperature above the eutectoid temperature. This grain boundary ferrite can
be described as proeutectoid ferrite to distinguish it from the eutectoid ferrite of the
eutectoid pearlite.
- Pseudomorph
- One phase copying the morphology characteristic of another phase. Typically,
as a result of corrosion, one material or phase mimics the form of the replaced
original. Pseudomorphic replacement of organic materials is common on iron
artefacts and can occur on copper alloys and silver/copper alloys as well.
- Proeutectoid
- When an alloy of composition away from the exact eutectoid composition is
cooled, particles of the first phase to precipitate are described as proeutectoid.
For example, on cooling a 0.6 % carbon steel from a temperature where the structure is
fully austenitic proeutectoid ferrite will start growing before the eutectoid reaction
produces pearlite.
- Puddling
-
As the production of coke pig for forge use took off from the 1740s onwards,
there were increasing attempts to find a cast-to-wrought iron conversion
process that was larger-scale and less labour-intensive than the finery,
and that obviated dependence on charcoal and water-power and the craft skill
of the finers. Many of these involved attempts to use the coal-fuelled reverberatory
furnace, either in conjunction with or as replacement for the finery, although,
the most successful process until the late 1780s, 'stamping and potting',
involved heating granulated iron (broken up by stamping) in sealed pots
with decarburising fluxes. In the late 1780s, however, Henry Cort's puddling
process was developed, and rapidly took over complete dominance; Cort's
main innovation was the use of grooved rolls to form the decarburised iron
into bars. In this process a charge of cold cast iron was melted in a reverberatory
puddling furnace with a sand bed. The air draught into the furnace was then
controlled to produce oxidising conditions, while the molten iron was stirred
with iron tools through the furnace door. As the carbon content dropped,
the iron became increasingly 'pasty', and was manipulated into large balls.
These were removed from the furnace, hammered and passed through grooved
rollers to consolidate the metal and form it rapidly into bars of uniform
section. The mechanical working of the iron at the various stages was an
important aspect of the process.
In Cort's process the cast iron reacted with silica in the sand bottom
to form a fayalitic slag. This resulted in a considerable loss of metal.
This so-called 'dry' puddling was later replaced by 'wet' puddling. In the
wet process, the floor of the furnace incorporated iron oxide in the form
of iron scale or roasted slag rich in iron oxides; also material rich in
iron oxides was added as part of the charge. The effect of this was to reduce
the losses of iron caused by slag formation and more importantly to aid
the decarburisation as the iron oxide reacted with the carbon in the molten
cast iron to form carbon monoxide. The wet process was also known as 'pig
boiling' from the strength of the reaction.
(Tylecote 1992, 126-129)
- Puddling furnace
-
A reverberatory furnace used for converting cast iron into wrought iron,
using a stirring or 'puddling' action in the later stages when the iron
becomes pasty.
Related terms - Cast Iron, Puddling,
Refining, Wrought
Iron
- Rabble
-
An iron rod bent at the end used to stir the molten cast iron
during puddling, to ball up the refined iron produced in the process
into blooms. The blooms would then be forged and rolled into bars of wrought
iron.
Probably, the term originally came from the French word - râble
, meaning rake. It was first used to describe the shovel used by charcoal
burners to tend the covering of the burning charcoal clamp, and was later
applied to puddling.
Related Terms - Puddling, Refining
(cast iron)
- Raising
The method of producing hollow-ware (dishes and bowls etc) by hammering
an initially flat sheet of metal over a curved stake. In raising the metal
is struck on the convex side of the form with a curved faced-hammer compressing
and thickening the walls of the vessel as the overall diameter of the vessel
is reduced and the edges are 'raised' above the centre. The diameter of the
initial blank is much larger than the diameter of the finished article; typically
the starting blank has a diameter equivalent to the combined dimensions of
the average diameter and the height of the finished form and is of relatively
thin sheet. The thickening of the metal at the rim is often increased further
by
caulking the edge. So that, starting
with a 0.9 mm thick blank the rim may end up 2 to 3 mm thick, and the vessel
is less likely to distort during use.
The raising process is usually preceded by a `blocking out` or `hollowing` process
in which the sheet is forged into a shallow dish-shaped form by hammering from what
will become the inside of the vessel against a concave depression in a wooden block.
Related Terms: blocking, caulking, dishing,
hollowing, sinking
- Reaction Solder
-
A method of soldering gold in which the joint was covered with a mixture
consisting of powdered copper compound, such as azurite, malachite or chrysocolla,
and an organic compound, then heated. The carbon rich compound reduced the
copper in the mixture to metallic copper. This copper dissolved in the gold
forming a small region on the surface of the join with a lower melting point.
If the temperature was high enough a solder (metal-liquid-metal) joint would
form. Prolonged heating would result in the copper diffusing into the body
of the gold, leaving little or no external evidence of the use of solder.
The technique was extensively used to produce Anglo-Saxon gold jewellery.
Sometimes the technique is misleadingly called diffusion soldering - the
solid-state diffusion of copper away from the join is a secondary effect.
Related Term - brazing, diffusion
bonding, solder
- Reamer
- A tool for cleaning out a hole bored by a drill, auger, or cast-in, in the case of hollow-cast objects.
Iron cannon were cast hollow until the 18th century, and finished by reaming the bore; a reamer with steel-edged cutters
has been recovered from Stream furnace, Sussex
- Recovery
-
The mechanical deformation (cold working) of a metal will increase its
strength, hardness, and electrical resistance, and will decrease its ductility.
Recovery is the term used to describe the tendency for these properties
to recover to the original values without any obvious change to the deformed
crystal grain structure. This may occur slowly at room temperature, or more
quickly as the temperature is increased.
Recovery is the first stage of the annealing cycle after cold working.
It occurs before recrystallization and grain growth. It occurs, largely,
through the elimination or rearrangement of the vacancies and dislocations
introduced into the crystal lattice of the metal during working. Many of
these processes follow an Arrhenius type law. That is, they are
time-temperature dependent with the rate of recovery increasing with temperature
in an exponential manner.
- Dynamic Recovery
- Dynamic recovery occurs when a recovery type process occurs during deformation.
It is often strongly temperature dependent and may extend the deformation
possible with some metals. But recrystallization, which occurs at higher
temperatures, is the main process that occurs during hot working.
- Recrystallization
-
Generally, a change from one grain structure to another with a lower
overall energy, without a change in crystal type. Recrystallization normally
occurs during an anneal after cold work, as it softens the metal so that
further working mechanical deformation can occur in the next cycle of cold
work. It is also the main mechanism by which the mechanical properties of the
metal recover during deformation in hot working.
Recrystallization is a nucleation and growth process, that
is, it is temperature controlled. In pure metals the rate of recrystallization
may become measurable at temperatures as low as a third of the absolute
melting point of the metal (0.3Tm). The presence of small quantities
of solute atoms can have a marked effect on the recrystallization temperature;
for example the addition of 0.01 atomic percent tin to high purity copper
will raise the recrystallization temperature by 180oC. With alloys
the recrystallization temperature is typically 0.5Tm
The final recrystallized grain size, for a given temperature,
is a function of the number of nucleation centres and the length of time that
recrystallization and grain growth has been possible, and the density of
pinning features such as second phase precipitates or inclusions. Nucleation
occurs in the regions of high lattice strain. These are near slip-line intersections,
deformation twin intersections, and the original grain boundaries. The
number of these features will increase with increasing deformation, but
at low strain there will be very few sites with sufficiently high dislocation
density to form nuclei for new grains. Thus the recrystallized grain size
tends to decrease with increasing deformation as nucleation predominates
over growth, whereas at low strains with few nucleation centres grain growth
predominates.
The aim of a recrystallization anneal is to complete primary
recrystallization but to keep the grain size as small as possible, as large
grain sizes are detrimental to the mechanical properties of metals.
- Recrystallization - critical strain
-
Below a critical strain, no or very few nucleation centres will
be created. Annealing metal that has not been deformed beyond the critical
strain will result in detrimental
secondary
recrystallization.
- Recrystallization - Primary
- Primary recrystallization is the term used to describe the replacement
of the original deformed grains with new grains. It is complete when all
the deformed original grains have been replace by new strain-free grains.
- Recrystallization - Secondary
- Once primary recrystallization is completed continued annealing, or
annealing at higher temperature than intended, will result in a second
wave of grain growth with some of the grains with favourable orientations
growing at the expense of others. In general, large grains will grow at
the expense of smaller. It is possible that boundaries of these grains will
not stopped by inclusions and second phases in the way that grains are
during primary recrystallization resulting in abnormally large grains.
- Recrystallization - Temperature
-
As recrystallization is a thermally activated process, the rate of
recrystallization roughly doubles with every 10oC. So there
is not, in fact, a fixed temperature at which recrystallization
starts, but because the process is so strongly temperature controlled,
below a certain temperature primary recrystallization is never
completed. The recrystallization temperature is usually defined as the
lowest temperature at which primary recrystallization is completed in
a reasonable time (typically 1 hour).
Related terms - Annealing, Grain
Growth, Grain Size, Recovery,
Strain
- Red Heat
- See Heat - red
- Reduction
-
Chemical reaction in which a metal is transformed from its oxide or sulphide
to the metallic form, or from one oxide to another with a lower number of
oxygen atoms for each metal atom. For example the series of transformations
-
| 6Fe2O3 = 4Fe3O4 + 02 |
Haematite to Magnetite |
| 2Fe3O4 = 6FeO + O2 |
Magnetite to Wüstite |
| 2FeO = Fe + O2 |
Wüstite to Metallic Iron |
the oxides of iron (going left to right) are successively reduced through
to metallic iron. Each individual stage is still a reduction, eg Magnetite
to Wüstite even when both reactant and the products are oxides.
Another way of looking at reduction is that it occurs when a cation
(the metallic ion) loses a unit of charge, e.g. Fe3+ goes to
Fe2+, or alternatively that the Fe ion gains an electron (the charge
on an electron is negative - e-)
Fe3 + e- = Fe2+
Related terms - Oxidization, Valency
- Refining
-
Refining is the separation of a metal from its impurities. As such it
is applied to a wide range of different processes for the different metals.
- Copper
-
The copper that was produced by the last stage of the smelting
process was, in general, rather impure (black copper with copper content
as low as 90%). This had a high iron content, which would have to
be reduced by two stage refining process if the copper was to be useable
for most purposes. This was done by first an oxidizing process to
convert the metallic iron in the metal to iron oxide (usually magnetite),
and then a reducing process (poling) to remove the excessive amount
of oxygen introduced by the iron removal stage.
- Refining black copper
-
The magnetite would float on top of the metal where it could
either be slagged by the addition of silica and tapped off, or
the pasty magnetite could be physically scrapped off the top of
the liquid metal to form a high magnetite smithing hearth bottom
type slag. Unfortunately, the tapped refining slag would be difficult
to distinguish from other of copper smelting slags, or even iron
smelting slag - however the presence of a significant amount of
partially digested crushed quartz fragments is often an indicator
of a copper smelting or refining slag.
The high magnetite smithing hearth bottoms, or skulls, have
been recorded rarely from archaeological contexts. This is almost
certainly because the number of sites on which refining occurs
would be limited as -
- Either the slag would be produced on the original smelting
site, where the small amount of such slag produced would be
swamped by the much large amount of 'normal' smelting slag.
Thus not easily recovered and recorded.
- Or the refining occurred away from the smelting site, that
is, the copper was traded as black copper ingots. In this
case, the slag might be mistaken as a slightly unusual iron
smithing slag.
- Poling
-
This was the final stage of the refining process designed to
reduce the oxygen content of copper to a reasonable level. It
was carried out by plunging a pole of wood into a bath of molten
copper this produces hydrogen and other reducing gases by the
distillation of the wood. These in turn reduce any copper oxide
present.
Alloying copper with tin or zinc would also deoxidise the metal
but at the expense of loss of expensive alloying element to slag
or vapour. It is, therefore, more economical to pole the metal
before alloying.
- Silver
-
The main method of separating silver from the majority of its impurities
was by
cupellation of the impure
metal with lead. However, cupellation would not separate gold from
silver.
- Gold
- Cupellation
-
Cupellation was used to separate gold from those alloys containing
copper but as silver is not oxidized during the cupellation process,
it would not separate gold from silver. Thus, if high purity gold
was required for jewellery, or coinage the metal would have to be
further refined by
parting.
- Mercury amalgam refining
-
In the classical period mercury amalgamation refining was used
to recover and refine gold from worn out gold embroideries and gold
leaf gilding. Gold dissolves in the mercury to form a pasty amalgam.
The majority of the mercury was remove by squeezing the mixture
chamois leather leaving the amalgam behind. The gold was then recovered
by heating amalgam so that the remaining mercury was lost by evaporation.
The same technique was later used to recover finely divided gold
from crushed ore.
-
Parting
-
To get rid of silver from gold it was necessary to use a parting
technique. The earliest of the heated the impure gold with salt
in combination with other chemicals to convert the silver to silver
chloride which diffused out of the metal. This method appears to
be first used to produce the gold for the Lydian coinage produced
at Sardis in the 6th century BC (
Rampage
and Craddock 2000).
Later other parting methods were used to treat impure gold alloys;
these included
acid, and the
antimony
or sulphide parting.
Relate term - Parting
- Iron
- Bloomery iron and steel did not undergo a specific refining process
other than by forging and possibly folding and welding to break up the
slag inclusions and homogenize the metal.
However, as the major use of iron was in the malleable form, the majority
of cast iron was (re)fined to wrought iron by oxidizing carbon, and
when present the silicon, out of the metal. Initially in Britain, this
was carried out in the finery forge
using the Walloon process where both
a finery and chafery
hearth were used. After Henry Cort's use of the reverberatory furnace
to convert the iron and grooved rolls to convert the resultant bloom
to bar, the puddling process generally replaced
the finery forge.
- Refinery
-
The refinery was a separate hearth used to convert grey cast iron into
white cast iron prior to its conversion to wrought iron in the finery and
chafery.
With the introduction of the coke fired blast furnace the
silicon content of cast iron increased. This resulted in the production
of grey cast iron rather than white, as was normally the case with the charcoal-fired
blast furnace. Grey cast iron was more difficult to refine, because the
silicon had to be oxidized before the carbon could be removed. The carbon
in grey cast iron was in the form of graphite was more difficult to oxidize
than when it was combined with iron in the form of cementite, as in white
cast iron. To solve this problem, an additional step in the fining process
was added, in which the grey cast iron was heated under oxidizing conditions
until the silicon was removed and the metal was converted to white cast
iron.
Related term - Chafery, Finery,
Puddling, Running-out fire
- Refractory
-
The term either describes the heat resisting properties of a material,
or the heat resistant objects such as tuyeres, crucibles, and furnace and
hearth linings. A refractory material must have the following properties.
- A high melting point
- It is likely that much of the history of metallurgy has been controlled
by the improvement of the properties of refractories. Although most
copper alloy metallurgy can be carried out in the temperature range
in which common clay-base refractories are adequate, the same is not
true for the production of steel and cast iron.
- Chemically inert with respect to the charge
- At high temperatures the slag produced can attack some furnace linings
extremely aggressively. This was particularly true in the 19th and 20th
centuries with the development of the basic steel making. The normal
silica refractories were attacked and destroyed rapidly by the basic
(calcium-rich) slag needed to reduce the phosphorus content of the steel.
- Mechanical strength and dimensional stability at high temperatures.
- In the case of crucibles, the material must be able to support the
weight of the molten metal and be able to be picked up without excess
deformation at the pouring temperature. In the case of furnace linings
they must be able to support the weight of the material above them without
slumping.
- Thermal shock resistance
- Crucibles, in particular, have to with stand large thermal shocks
when the crucible is removed from the furnace for pouring. Furnace linings
have to withstand thermal stresses imposed by thermal gradients within
the furnace lining.
- Thermal conductivity
-
Ideally crucibles and muffles should have high conductivity, whereas,
furnace linings should have low conductivity. However, conductivity
of crucible refractory was not factor that would have been considered
important until recently, the other factors being much more important.
Similarly, for furnace linings the requirement for low thermal conductivity
was not an overriding factor, although reducing the overall thermal
losses through the furnace walls would have been important.
The difference between the thermal conductivity of modern refractories
and those used in the past has not always been considered in some
experimental reconstructions of early smelting processes. The use
of different refractory materials can have a major effect on the thermal
losses and thus on the blowing and rate of fuel use required to run
a smelt.
The earliest refractories were made from local clays. These may have
been modified to improve their properties usually by the addition of quartz
rich sand, but where available graphite was used. Eventually, some clays
(high kaolin fire-clays) were discovered to make particularly good refractories.
So that production became centralized in a few centres and the crucible
exported over large distances as is shown by the import of large number
of Hessian crucibles, manufactured in what is now Germany, into England
during the 17th and 18th centuries. The introduction of the blast furnace
required the use of refractories with high silica content to withstand the
higher temperatures involved. These were either in the form of natural high
purity sandstones with low iron content, or later silica bricks that contain
more than 96% silica. To deal with basic slag or higher temperatures refractories
based on magnesite MgO or dolomite (Ca.Mg)CO3 were used.
- Regenerative Heating
-
A regenerative heater was a fuel saving device fitted to a furnace, consisting
of two sets of chambers filled with a checker-work of firebricks. One chamber
is heated by the hot waste gas from the furnace, while the other, heated
in the preceding cycle, is used to heat the incoming cold air and gas(fuel).
After a period the air flow through the furnace and chambers is reversed,
so that the chambers change their rôles.
The idea was based on the idea patented by Stirling in 1812, and fully
developed by Frederick Siemens between 1856 and 1861. The earlier versions
used coal and other solid fuels, but there was a tendency for the ash carried
over by the airflow to block the firebrick checkers. The use of producer
gas as a fuel eliminated this problem. The use of regenerative heaters allowed
the furnaces, both to run using less fuel, and at higher temperatures.
William Siemens applied the use of the regenerative heater to steel making
and developed the
Open Hearth method of steel
making, which was used extensively in Britain
Related term - Hot blast
- Relief Polishing
- During sample preparation for metallography, there is a tendency for the
softer phases in a sample to be preferentially removed by the abrasive paste.
Normally this effect is not wanted, but may be helpful in revealing phosphorus
ghost structures in phosphoritic irons.
- Repoussé
-
A design is raised on the front side of the metal by hammering from the
back surface. The front side is supported with a soft material such as pitch
during the repoussé stage, so that large areas of metal compared
with the thickness of the sheet was deformed (unlike chasing).
- Retained Austenite
-
With high-carbon steels the Mf temperature can be reduced
to well below ambient temperatures. If such a material is not tempered after
quenching, some of the structure will remain as austenite. Retained austenite
is rarely seen in examples of archaeological or historical steels as most
early steels are either not quenched sufficiently fast, or are tempered.
However, retained austenite has been observed in a few Medieval Iranian
ultra-high carbon steels.
- Retort
- A vessel with a long turned down neck used to distil materials by volatilisation.
Originally, applied to chemical apparatus, but also furnaces in which metals
were purified or smelted by volatilisation - mercury and zinc. The term was
also applied to the oven in which coal gas was heated to produce coal gas
and coke.
- Reverberatory furnace
-
The name comes from the verb, to reverberate - to beat back or reflect,
thus, a reverbatory furnace is one in which the flame or heat is reflected
back on to the charge.
This has the effect that fuel is separated from the charge as the heat is
reflected down onto the charge from the roof. The furnace is of the induced
draught type with a tall chimney at one end and a firebox at the other. It
needs a long-flame fuel such as coal or wood (not charcoal or coke) to transfer
the heat towards the near-horizontal roof. Used for remelting cast iron, bronze
and bell metal. Often known as an 'air furnace' and latterly used for puddling,
steel-making and non-ferrous smelting.
- Riser
- When metal is poured into a mould air has to be displaced. A riser is a
vent that lets out any trapped air, and thus, allows the metal to enter the
mould more easily. Generally, situated on the opposite side of the mould from
the feeder. When the metal is seen in the riser, pouring is stopped. The use
of risers appears to be a late development in casting technology as the is
no evidence for the use of risers with early stone and metal moulds, or clay
investment or part-moulds.
Comment The introduction of the use of risers must have been associated
with the development of large castings requiring the use of casting pits.
However, although the feeder arrangement of moulds is often discussed in the
archaeometallurgical literature, the use of risers is rarely discussed or
illustrated.
- Riveting
-
The method of joining metal sheets by metal pegs passing through aligned
holes previously cut in the sheets and hammering down the ends. Riveting
can be performed hot or cold. The term hot or cold referring to the temperature
of the rivet when it is closed. The advantage of hot riveting being that
the joint is tightened further by the thermal contraction of the rivet on
cooling.
- Rolling
-
The hand-powdered rolls were first used to emboss narrow strips for coins
and to flatten and thin soft metals like gold and lead. But the development
of the machinery capable of thinning work hardening metals and alloys required
the improvement of the materials for the rolls and the their supporting
structures as well as the increased power available in the post-medieval
period.
It is likely that the early rolling mill technology came from the sltting
mill, and
Tylecote 1990 (249) quotes C.S.
Smith's reference to the earliest description of a rolling mill of 1568
for the preparation lead window 'cames'. Clearly the use of rolls to form metal was
of interest at this period, as in 1496 Leonardo da Vinci drew a design for a rolling mill.
Related terms - Slitting
- Roasting
-
Another term used for the heating of materials either in open heaps or
simple kilns. Ores may be roasted either under oxidising conditions, or
very limited reducing conditions. Roasting was used to drive off water (both
any chemically combined water, as well as any general dampness), decompose
carbonates, and break up the ore to make it more permeable to gases. Roasting
may be used to partially convert sulphides to oxides prior to forming a
matte.
Related terms - Calcine, Dead
Roast.
- Rockwell Hardness Test
-
The Rockwell hardness testing method uses either a steel ball (Rockwell
B) or a diamond cone having an angle of 120o (Rockwell C). It
is used for rapid testing but the results are not directly comparable to
Vickers Pyramid numbers. A disadvantage of the Rockwell test is single test-head
cannot be used over the whole range of materials from soft copper and brass
to very hard materials such as nitrided surfaces.
- Runner
-
In a casting the runner is a passage connecting the sprue to the ingate
of the mould. A runner is normally horizontal or near horizontal, and helps
control the flow of the metal into the mould.
- Running-out fire
-
The introduction of coke-fired blast furnace meant that the iron produced
had a higher silicon content than was the case with the charcoal-fire blast
furnace. As silicon is more reactive than carbon, it protects the carbon
from oxidization until most of the silicon has been removed, and thus extending
the time required to complete the refining process. In addition, high silicon
cast irons tend solidify as grey cast iron rather than white. Grey cast
iron is more difficult to convert than white iron. The running-out fire
was a method of overcoming these problems. It was a hearth into which the
liquid iron from the blast furnace could be tapped. It had a series of tuyeres
that blew air down on to the surface of the pool of metal oxidizing the
silicon to slag; the resulting low silicon could then be converted as before.
Related terms - Finery, Puddling,
Refinery.
- Rutile
- Titanium dioxide. TiO2. A common accessory mineral in many rocks,
in particular it is associated with placer magnetite iron ores.
Chris Salter, Oxford Materials Characterisation Service.