Talc (Mg₃Si₄O₁₀(OH)₂) is very interesting, particularly for cyclic fabrication systems, because it calcines to enstatite and amorphous silica at 800–840°, and the enstatite (MgSiO₃) converts to clinoenstatite at 1200° (?), and the amorphous silica to cristobalite at 1300° (or possibly it converts to enstatite above 1050°?); enstatite melts at some 1550° and quartz/cristobalite at 1713°. You could think of enstatite as being a quartz/forsterite eutectic, and indeed 1543° is the lowest-melting point in the quartz/forsterite continuum, but the cristobalite crystals remain solid until 1713°. (I’m not sure how solid the resulting slush is, though...)
If you have an ample supply of talc, then, as with clay, you can shape it into the geometry you want, and then fire it to harden it. As with clay, you can use tools thus made to shape new objects. However, firing talc to harden it does not (?) cause it to become plastic and change in shape or shrink; the results are nonporous and extremely unreactive; and they are capable of withstanding much higher temperatures than those needed to fire them, so it should be easy to make talc-firing ovens out of fired talc. The required firing temperature is lower than for most clays, and much lower than for high-performance porcelain clays.
Because the unfired talc is not plastic and does not become plastic during firing, very large shapes can be precisely cut and fired in a gravitational field without suffering plastic deformation. Though brittle, the fired objects are highly resistant to thermal shock, in part because of their low thermal coefficient of expansion (enstatite’s linear TCE is a steel-like 11 ppm/K), permitting their use for a wide variety of applications with demanding temperature gradient requirements.
Talc is a useful feedstock for synthesizing cordierite, a common refractory with an acicular crystal habit and even lower TCE, which can be as low as 0.8 ppm/K for porous cordierite materials.
For machinery involving sliding contact, unfired talc serves as a dry lubricant.
Obtaining adequate supplies of natural talc may be difficult, but straightforward hydrothermal talc synthesis processes are known, and some of them can be carried out in vessels made of fired talc. The raw feedstocks are abundant: oxygen is 47% of the Earth’s crust, silicon 28%, and magnesium 2.1%. Only hydrogen is scarce (0.14%), and it is removed in the firing process.
Fired talc is hard enough to grind or cut softer metals, but I think it can only barely cut steel, if that.
Even fired, talc is not a very good material for withstanding impact, controlling electricity, or spring energy storage.
An anonymous stone sculptor writes that they have been firing soapstone as a stone-sculpting technique for many years:
Many years ago I wrote an article about firing soapstone to make the stone harder. It is possible to harden the stone to the point that you cannot scratch it with anything short of steel. This will also close the structure of the stone enough to allow it to be used in outdoor applications. ... Insulators on the electric poles were at one time made of fired soapstone.
A geology.com article says the Americans have been carving soapstone for 8000 years, although I’m not clear on whether the results were fired — but because cooking bowls, cooking slabs, and smoking pipes were among the uses, I suspect they were. The same article mentions the Scandinavian Bronze-Age use of soapstone for carving bronze-casting molds. (Also, it mentions that some “soapstone carvings” are actually alabaster or serpentine.)
Reportedly potters use 50% talc, 50% ball clay for slipcasting.
In “Synthesis of MgSiO₃ ceramics using natural desert sand as SiO2 source” Wang et al. mention some pretty cool benefits of enstatite-based ceramics, including protoenstatite’s “adjustable coefficient of thermal expansion (CTE), high strength, low dielectric loss, and strong vibration resistance” at high temperatures, but lament its tendency to crack when transforming to clinoenstatite at lower temperatures. Evidently this is not a problem with fired-talc ceramics, though.
Apparently if you react soluble magnesium and silicate salts, at low enough Mg/Si ratios you get crystalline talc but only above 200°. But, according to the same paper, it looks like amorphous synthetic magnesium silicate hydrates also produce silica when fired in the same way, but at a slightly lower temperature:
Differential thermal analysis (DTA) of the M-S-H samples reveals a distinct exothermic transition around 840 - 860 °C, which is attributed to the decomposition of amorphous M-S-H and the recrystallization to SiO₂ and enstatite (MgSiO₃) [11, 12] or SiO₂ and forsterite (Mg₂SiO₄) for high Mg/Si ratios [29]. The dehydroxylation of the phyllosilicate sheets in talc (Mg₃Si₄O₁₀(OH)₂) and the transformation to SiO₂ and enstatite occurs at slightly higher temperatures of 850 to 950 °C [11, 12]. The lower dehydroxylation temperature of M-S-H compared to talc is consistent with a less crystalline structure [11, 12].
(It’s worth noting that their room-temperature synthesis took a year, but only because they were using low-solubility feedstocks.)
And I’ve prepared what seems to be this amorphous material in my kitchen by dumping magnesium chloride from the health food store into a can of waterglass (see Glass foam for details). However, it came out in the state of sort of slushy lumps; I’m not sure how to stick it together into a more solid material. Maybe with more waterglass, I don’t know.
https://usenaturalstone.org/soft-spot-soapstone/
https://doradosoapstone.com/blog/6-fun-and-historical-facts-about-soapstone/ notes on firing
https://www.golcha.com/insulators.html talc and clay electrical insulators
https://wmblogs.wm.edu/sgtresearch/the-soapstone-special/ mining in Virginia
https://en.wikipedia.org/wiki/Insulator_%28electricity%29#Insulation_of_antennas steatite mountings
https://www.jpcfrance.eu/technical-informations/electrical/historical-introduction-of-ceramics-used-in-connection-blocks/ notes on historical electrothermal applications
https://isolantite.com/ “Isolantite has been a major producer of precision Steatite Ceramic electrical components for over 100 years”
https://www.r-infinity.com/Companies/ history of insulator companies
https://www.lspceramics.com/steatite-ceramic-insulators/ “We can grind steatite ceramic insulators to your specs, achieving tolerances within 0.0005”.”
https://www.indiamart.com/proddetail/anoop-steatite-and-alumina-ceramic-insulators-1460976162.html “used in all types of communication devices operating at a very high voltage and high frequency”
https://www.sciencedirect.com/science/article/pii/S027288421930906X “Synthesis of MgSiO3 ceramics using natural desert sand as SiO2 source”
https://www.sciencedirect.com/science/article/pii/S0272884206002227 “Synthesis and characterization of MgSiO3-containing glass-ceramics”
https://www.tandfonline.com/doi/abs/10.1179/174367606X128423 “Microstructure, mechanical and thermal properties of boron oxide added steatite ceramics”
https://www.tandfonline.com/doi/abs/10.1080/0371750X.2019.1657954 “Production and Characterization of Alumina and Steatite Based Ceramic Insulators”
https://www.sciencedirect.com/science/article/pii/S0272884206000861 “Low-temperature fabrication of steatite ceramics with boron oxide addition”
Steatite ceramics have been fabricated by using coarse starting materials such as talc, clay, and barium carbonate with addition of boron oxide (B2O3). B2O3 has been found to be a useful flux to densify MgO–Al2O3–SiO2–BaO (MASB) powders. The steatite ceramic with a relative density of 97% was obtained at a sintering temperature of 1200 °C.
https://www.sciencedirect.com/science/article/pii/S0272884218329262 “Effects of mechanical-activation and TiO2 addition on the behavior of two-step sintered steatite ceramics”
Steatite, as ceramic with composition predominantly resting on magnesium silicate, was produced from economic resources – talc, aluminosilicate clays, and either BaCO3 or feldspar as flux.
https://www.degruyter.com/document/doi/10.3139/146.110409/html "The role of talc in preparing steatite slurries suitable for spray-drying"
https://link.springer.com/article/10.1007%2FBF01285830 “Sintering corundum with additives”, 01964, Ukraine Scientific-Research Institute of Refractories, USSR
https://ieeexplore.ieee.org/abstract/document/5781588 “Notice of Retraction: Synthesizing Cordierite from High-Alumina Fly Ash and Talc Powder”
https://www.degruyter.com/document/doi/10.1515/chem-2020-0154/html “Synthesis of magnesium carbonate hydrate from natural talc” cc-by. kind of backwards from what I’m looking for tho...
https://www.nature.com/articles/srep22163 “Rapid growth of mineral deposits at artificial seafloor hydrothermal vents” mostly sulfide with a little talc
https://link.springer.com/article/10.1007/s00410-009-0395-4 “Growth and deformation mechanisms of talc along a natural fault: a micro/nanostructural investigation”
https://pubmed.ncbi.nlm.nih.gov/28771845/ “Synthetic Talc and Talc-Like Structures: Preparation, Features and Applications”
This contribution gives a comprehensive review about the progress in preparation methods, properties and applications of the different synthetic talc types: i) crystalline nanotalc synthesized by hydrothermal treatment; ii) amorphous and/or short-range order nanotalc obtained by precipitation, and iii) organic-inorganic hybrid talc-like structures obtained through a sol-gel process or a chemical grafting. Several advantages of nanotalc such as high chemical purity, high surface area, tunable submicronic size, high thermal stability, and hydrophilic character (leading to be the first fluid mineral) are emphasized
https://www.nature.com/articles/ncomms10150 “Talc-dominated seafloor deposits reveal a new class of hydrothermal system”
https://www.chemistryviews.org/details/ezine/9489511/Talc_from_the_Lab.html “In the journal Angewandte Chemie, French scientists introduce a laboratory synthesis of talc, which needs only seconds and produces nanocrystals with unique properties useful in many applications ... Two research groups under the direction of Cyril Aymonier and François Martin in Bordeaux and Toulouse, France, have developed a method to produce synthetic talc nanocrystals hydrothermally in supercritical water.”
https://pubs.rsc.org/en/content/articlelanding/2010/jm/c0jm01276a#!divAbstract “Functionalization of synthetic talc-like phyllosilicates by alkoxyorganosilane grafting”
A range of talc-like phyllosilicates were prepared via a hydrothermal synthesis performed at five different temperatures from 160 to 350 °C. The organization of the lattice and the degree of crystallinity of the new materials were evaluated by different techniques such as XRD, FTIR, solid-state 29Si NMR, TEM, FEG-SEM and TG-DTA. When synthesized at low temperature the material presents high degree of hydration, low crystallinity and flawed structure. This was attributed to stevensite-talc interstratified product present in the samples. The stevensite/talc ratio and the hydration decrease in the talc-like phyllosilicate samples when the hydrothermal synthesis temperature increases and so the crystallinity becomes higher. A thermal treatment at 500 °C allowed a significant flaw reduction in talc-like phyllosilicate structure; the synthesized sample at 350 °C and heat treated presents a structure close to that of talc.
https://www.sciencedirect.com/science/article/abs/pii/S0169131713003001 “Synthetic talc advances: Coming closer to nature, added value, and industrial requirements”
Over the past 2 years, the synthetic process of talc particles has evolved considerably, leading to an inexpensive, convenient, and rapid process that is compatible with industrial requirements. In addition to facilitate the synthetic talc preparation, the evolution of the synthesis process has led to an improved crystallographic arrangement of the talc particles in both the c* direction and (ab) plane. In the present study, the most recent process was investigated with respect to the reaction time, temperature, pressure, pH, and salt concentration to determine the optimal reaction parameters.
https://egyptmanchester.wordpress.com/2018/01/19/the-use-of-steatite-in-ancient-egypt/
In its raw state the softness of steatite make it extremely easily damaged, and simply wearing or using a carved object would damage the carved detail. Steatite has an interesting property, when it is fired it will convert from steatite into enstatite. Unlike steatite, enstatite has a hardness of Mohs 5.5 which is close to that of granite – making it extremely hard wearing and resistant to damage, whilst still retaining its carved detail. Steatite has also been glazed since the Predynastic era for objects such as beads and amulets. ... Firing at a temperature of ~950°C will cause steatite to dehydrate and crystallise into enstatite. Clay will begin its vitrification process ~800-900°C and firing will generally require temperatures in excess of 1100°C , therefore the steatite to enstatite conversion can be achieved using similar technology as is required for firing clay objects. A wood fuelled open fire can easily reach temperatures exceeding 1100°C, and can be used for firing ceramics and also for converting steatite to enstatite.
Further reading: Connor, S, Tavier, H and De Putter, T. ‘Put the Statues in the Oven: Preliminary Results of Research on Steatite Sculpture from the Late Middle Kingdom’. Journal of Egyptian Archaeology 101 (2015).
https://www.hindawi.com/journals/geofluids/2017/3942826/ "An Experimental Study of the Formation of Talc through CaMg(CO3)2–SiO2–H2O Interaction at 100–200°C and Vapor-Saturation Pressures"
In this study, in situ Raman spectroscopy, quenched scanning electron microscopy, micro-X-ray diffraction, and thermodynamic calculations were used to explore the interplay between dolomite and silica-rich fluids at relatively low temperatures in fused silica tubes. Results showed that talc formed at ≤200°C and low CO2 partial pressures (PCO2). The reaction rate increased with increasing temperature and decreased with increasing PCO2. The major contributions of this study are as follows: we confirmed the formation mechanism of Mg-carbonate-hosted talc deposits and proved that talc can form at ≤200°C; the presence of talc in carbonate reservoirs can indicate the activity of silica-rich hydrothermal fluids; and (3) the reactivity and solubility of silica require further consideration, when a fused silica tube is used as the reactor in high P–T experiments.
https://www.sciencedirect.com/science/article/abs/pii/S0169131715001532 “Technological properties of ceramic produced from steatite (soapstone) residues–kaolinite clay ceramic composites”
Ceramic bodies (7.0 cm × 2.0 cm × 1.0 cm) of kaolinite clay and soapstone residuals collected from workshops in Ouro Preto and Mariana, Minas Gerais, Brazil, containing from 2.5 to 97.5 wt% steatite (soapstone) were prepared and firing at 500, 1000 and 1200 °C, for 2 h, in air. The linear shrinkage, compressive strength, water absorption and mass loss by heating were determined on the samples after heat treatment. The fired samples at 1000 and 1200 °C, with steatite percentages of 85, 90 and 95%, presented the best results for technological applications in ceramic industry. For these samples, the values of the compressive strength were higher than 10 MPa and those of water absorption varied between 8 and 22%, which means that the values of these properties are superior and inferior, respectively, to the reference values established by Brazilian Standards. The linear shrinkage was lower than 6%, which is the maximum value established by the Pólo Cerâmico de Santa Gertrudes, in São Paulo State.
https://experimental-prehistory.blogspot.com/2016/02/soapstone-steatite-as-hard-as-flint.html “Making Soapstone/Steatite as hard as flint ? after firing, at ~950°C?”
https://www.mdpi.com/2075-163X/8/5/200/htm “Looking Like Gold: Chlorite and Talc Transformation in the Golden Slip Ware Production (Swat Valley, North-Western Pakistan)”
...In order to constrain the firing production technology, laboratory replicas were produced using a locally collected clay and coating them with ground chlorite-talc schist. On the basis of the mineralogical association observed in both the slip and the ceramic paste and the thermodynamic stability of the pristine mineral phases, the golden slip pottery underwent firing under oxidising conditions in the temperature interval between 800 °C and 850 °C. The golden and shining looks of the slip were here interpreted as the result of the combined light reflectance of the platy structure of the talc-based coating and the uniform, bright red colour of the oxidized ceramic background.
https://digitalfire.com/material/talc
http://webmineral.com/data/Clinoenstatite.shtml#.YO9psLOdJhE
https://www.mindat.org/min-1072.html clinoenstatite
https://www.sciencedirect.com/science/article/pii/S2187076415000652 “Talc-based cementitious products: Effect of talc calcination” cc-by-nc-nd
This study reports the use of calcined talc for cementitious products making. The calcination is used to enhance the availability of magnesium from talc to react with phosphate for cement phase formation. It is shown that previous calcination of talc leads to products having enhanced mechanical performance due to the formation of more cement phase than in products based on raw talc. Talc fired at 900 °C was found to be the one in which magnesium release was maximal. Firing at temperature higher than 900 °C leads to the stabilization of enstatite, which decreased the magnesium availability. The cement phase is struvite, which was better detected on the X-ray patterns of the products involving fired talc. All the products have very rapid setting time and low shrinkage.
https://www3.epa.gov/ttnchie1/ap42/ch11/final/c11s26.pdf talc processing
https://nvlpubs.nist.gov/nistpubs/jres/15/jresv15n5p551_A1b.pdf
https://rruff.info/doclib/hom/clinoenstatite.pdf
http://deanpresnall.org/files/90Enst.melting.pdf
http://www.geo.umass.edu/courses/geo321/Lecture%208%20Binary%20Systems.pdf
https://hal.archives-ouvertes.fr/hal-01686526/file/ACL-2018-009.pdf
https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.643.1922&rep=rep1&type=pdf
http://www.doiserbia.nb.rs/img/doi/0354-4656/2018/0354-46561803297R.pdf
http://przyrbwn.icm.edu.pl/APP/PDF/127/a127z4p077.pdf
https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.537.1305&rep=rep1&type=pdf
https://www.gtcountymi.gov/AgendaCenter/ViewFile/Item/1198?fileID=5819
https://www.repositorio.ufop.br/bitstream/123456789/8834/1/ARTIGO_TechnologicalPropertiesCeramic.pdf
https://www.ceramics-silikaty.cz/2015/pdf/2015_04_331.pdf