Soapstone / Steatite as hard as flint ? after firing at ~950°C ?

Soapstone / Steatite is marvelously easy to carve... and it can become hard and durable. Like clay, it has to be fired. Apparently 1, 2 or 3 hours at ~950°C.

This is definitely worth a try. With very primitive means.
Apparently the only technological requirement is knowing how to make big fires.

A few links and extracts :
"Native steatite is so soft it can be scratched with a fingernail, but baking results in dehydration and hardening of the stone. Some ancient steatite carvings were glazed then fired which produced a mineral (enstatite) hard enough to scratch glass."
"[...] the  so-called  Harappan script [... signs ] have  the  most  peculiar  and  elaborate iconographies  of  Indus  Civilization.  It  is  represented  on  many  stamp  seals  of  fired  steatite  and  corresponding clay  sealing,  terracotta tablets [...]"
"A group of 16 three-sided incised baked steatite tablets, all with the same inscriptions, were uncovered in mid- to late Period 3B debris outside of the curtain wall."
"[...]Seals of the Indus civilization were generally made from two different materials, steatite and faience.[...] Steatite [...] was hardened by firing at a high temperature. [...] We know that steatite beads and ornaments were first carved while the rock was soft. The finished object was then heated in a kiln to over 900° C to harden [...] Faience was also fired in a kiln that reached temperatures over 900° C to partially vitrify the powdered quartz and to create a vitreous glaze on the surface [...]"

Experiment made in 2001, Madison, Wisconsin ; 935°C after 3 hours of firing and maintained 1 hour by continuously fueling wood.

From that, I understand that no kiln were used, but only "canisters" and "setters", put into a pile of wood. Frustratingly, the article shows no drawing or photos of that experiment, so the reader is left alone to imagine what exactly the description refers to...

At least there is another page,
showing a few photos of "canisters" and "setters"... the way to put the "setters" in place is not clear; and the way they put 2 canisters one on top of the other seems odd and unstable (no suprise that impurities did enter into it).
google image search, : images of egyptian glazed steatite
Modern firing of soapstone, described; warnings about inclusions that can break a stone
Not very hot... but long... no hints about the hardness achieved : "to a high of no more than 500 degrees"; 24 hours of gradual heating, then 8 more hours.
[about ancient beads from the sultanate of Oman] "We propose that steatite was used in the manufacturing process of the beads. The beads might have been made from soft steatite material and then hardened by firing at about 1000°C; the formerly soft beads thus became hard and durable. This hardening is due to the transformation of the steatite into synthetic enstatite which takes place at about 1000 °C."
(in french) Says that, around 1750 BC, Egyptians who were not rich enough had small sculptures made of steatite/sopastone, then baked at 900°C or higher so that they became as hard and durable as the statues made - from hardstones - for richer people.
« Cuire des statues » : l’usage de la stéatite en sculpture à la fin du Moyen Empire et à la Deuxième Période Intermédiaire.
"[...] Une cuisson à 900° C permet en effet de la durcir considérablement, de la rougir ou de la noircir (selon l’atmosphère de cuisson) [...] Sous l’effet de la chaleur, la pierre se déshydrate et, à partir d’une température de 900° C, se transforme en enstatite, d’un indice de dureté de 7 sur l’échelle de Mohs (au lieu de 1 pour la stéatite à l’état naturel) . Cette propriété est aujourd’hui utilisée par les artisans de la rive ouest de Louqsor pour produire des imitations de fritte, en enduisant d’une préparation à base de poudre de cuivre les pièces qu’ils ont réalisées en stéatite. Après cuisson à ciel ouvert, les reproductions d’amulettes et d’hippopotames sortent du foyer bleues, brillantes et très dures. [...]. Un simple four de quelques dizaines de centimètres de diamètre, creusé dans le sol, alimenté par un alandier et recouvert de briques a permis d’atteindre la température de 900° C en moins d’une heure. Un combustible particulièrement efficace et certainement aussi le moins onéreux pour cette opération s’est révélé être la bouse de vache séchée (utilisée encore aujourd’hui dans les campagnes égyptiennes). Lors d’une cuisson à ciel ouvert, en atmosphère oxydante, la stéatite rougit et obtient une couleur brun-rouge orangé, que l’on retrouve sur certaines statuettes de la XIIIe dynastie [...]. C’est probablement surtout une cuisson en atmosphère réductrice qui devait être utilisée. En effet, en refermant au bout d’une heure le four et en laissant se poursuivre la cuisson de la stéatite en enfumage pendant toute une journée, la surface obtenue était bien noire et dure. Un simple polissage a suffi ensuite à conférer à la pièce l’aspect luisant des statuettes du Moyen Empire."
(in french again) In the 18th century, artists were carving cameo from steatite/soapstone, then they baked them (to "red white" color) during 2 to 3 hours so that they become has hard as flint. They also had various ways for coloring the resulting stones. The paper also reports another experiment : making a paste with water and steatite/soaptsone powder, bake it (like clay), in hope of getting some hard rock : didn't work.
"[... about small beads made from soft talc, then heated] The temperature at which enstatite crystallises is above 1200 C. This technology resembles that known from ancient India and Iran."
"[...] and other figurines (burials 634-638) made from a white heated form of steatite [...]"
The effect of firing temperature on properties of natural steatite and pyrophyllite
[The abstracts gives no practical new hint useful for prehistoric technology re-enactment : steatite hardens when heated above 900°C]
"Both materials were fired in hydrogen and air, over the temperature ranges of 950 to 1100° C and 1000 to 1150° C for the steatite and pyrophyllite, respectively. [...] The strength and hardness of the hydrogen-fired pyrophyllite was higher than its air-fired counterpart. The opposite result was observed in the steatite."
a modern industrial process for firing steatite (but no information about the hardness obtained)

Cultural and social dimensions of the prehistoric Gulf Island soapstone industry - 1994 - Inge R. Dahm
Very interesting (artifacts description, culture, 200+km long distance exchanges, tools for carving soapstone, ...).
Apart from a few remarks like "this artifact appears to have been burnt", I couldn't find any discussion about a possible heat treatment of the soapstone artifacts and ornaments. Although this is a question that every reader would like to see answered : were those fragile artifacts hardened in some way, like in various other cultures, to withstand years of usage and... thousands of years in the ground ?
This being said, for many of this artifacs, especially labrets, softness may have been prefered to hardness: you don't want your teeth to be broken by a too hard stone labret. And the author regularly underscores the softness of the stone : maybe he verified it on most artifacts (? ... I didn't find any information about chemical analyses or hardness test).
About about physical properties, said to be "relevant to prehistoric soapstone carving", one information is the softness of the stone. And the only other information, rather irrelevant in prehistoric context is : "Fusability : Reaction to heat treatment. Soapstone will whiten and exfoliate when heated alone before a blowpipe and fuse to an enamel on the edges only". A few lines after that, another information that misses the point : "Soapstone can be heat treated to a hardness of 8 but will become brittle and will fracture or shatter on impact". Not very useful, because what we need to know is the amount of heat treatment - achievable with primitive means - that would confer some durability to (what variety and quality of) soapstone; and not the result of what appears extreme heat treatment (in other documents, heat treatments are said to harden soapstone to a hardness around 5 or 6).

Encyclopaedia Americana - 1800 - 1872 
On page 583 (579) , about Steatite... here is a pinnacle of imprecision : "exposed to heat it, becomes much harder". Other nevertheless interesting information : "the variety of  steatite called potstone is in hardness nearly the same as common steatite, but is more tenacious. [...] Steatite is not susceptible of a good polish; but its softness and tenacity [...] and its property of becoming hard by heat, render it a useful mineral in the arts. [...] The common steatite has even been employed for the purpose of engraving ; for, beeing easily cut when soft, it may be made to assume any desired form, and afterwards rendered hard by heat. It then becomes susceptible of a plolish, and may be variously colored by metalic solutions"
[protected content]
Glazed Steatite: An Investigation of the Methods of Glazing Used in Ancient Egypt  (Jun., 1989)
"Whichever glazing method is used for steatie [...] the body becomes very much harder"
[Talc Investigation in Vermont - Preliminary report - 1951 - US departement of the interior - geological survey]
page 2 : "The pure, dense, crypto-crystalline variety of steatite known as "lava grade" is valuable because it can be machined into intricate forms and then heat-treated to great hardness with negligible shrinkage."
 "Lava-grade block talc is a variety of steatite dis- tinguished by its suitability for machining. It must be free of flaws and must not crack during firing. Formerly, lava-grade block talc was the only type of steatite used for insulators. Now, however, it is employed only in rela- tively small quantities for spacers in radar vaccuum tubes and for other specialized purposes. Since the discovery in the early 1920 's that pulverized talc could be used for making insulators, most steatite has been ground before firing. It is mixed with a binder, and pressed or extruded into the required shapes."
[Letters Patent No. 71,919, dated December. 10, 1867. Henry Julius Smith, Boston]
"put the articles so formed in intimate contact with some kind of carbon, (I prefer finely-ground hone-coal or plumbago,) into a closed vessel, of suitable material for resisting heat, and submit the vessel and contents to heat for an hour or more, removing the same when a white heat has been reached. The articles are then found to be hard and tough. "
This corresponds probably to a temperature almost impossible to reach by prehistoric means.

No comments:

Post a Comment