|Monthly Tech-Tip |
Flameware is ceramic that can withstand sudden temperature changes without cracking. The low thermal expansion of true flameware makes craze-free glazes very difficult.
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Flameware is ceramic that can withstand sudden temperature changes without cracking (i.e. stove top burners). Ovenware is another class of ceramics, it is not as resistant to thermal shock as flameware. There is some confusion among clay buyers and retailers about this. For example Japanese Donabe ware is sometimes touted as flameware but the claims and precautions recommended for its use show that it is ovenware (e.g. requirement to be water-logged and not heated without contents, claim of resistance to failure at high temperatures which is completely different than resistance to sudden temperature changes).
Ceramic is very susceptible to thermal shock failure because of its brittle nature (with accompanying lack of elasticity and tendency to propagate cracks). Brittle matrixes just cannot absorb the stresses that occur when sudden heating or cooling imposes expansion or contraction in one part or section of a piece at a different rate than another. After reading this page you may feel you have received little help for a flameware recipe. But it is possible to greatly enhance the ability of a piece to withstand shock, it is a combination of body recipe, firing, ware shape and contour and modifying the manner in which heat is applied.
Indigenous cultures have made terra cotta cooking vessels for use over an open fire for thousands of years. Their secret is simply the high porosity of the fired material. While the open structure and lack of a glassy phase produce low strength they also enable much lower brittleness. There is enough micro-mobility within the matrix to absorb the expansion gradients of sudden heating and the open structure terminates micro-cracks. They also do not glaze the ware (although leaded glazes of sufficiently low expansion can be workable if applied thinly). In addition, people who use this type of ware are tolerant of cracks that form, its low strength and gradual disintegration during use and they know how to be careful in handling it.
With vitreous flameware failure occurs in a rather more spectacular fashion. High proportions of low-expansion materials like kyanite, mullite, pyrophyllite or molochite (powder or grog) can be plastic-bonded with a small amount of clay or organic binder and fire-bonded with a low expansion flux. Of course, if the kiln heat is high enough so that particles of these materials are altered to a different form or are taken into solution in the glass bonder during firing (e.g. feldspar) then the low expansion character of their natural state can be partly or fully lost. Even if a low-expansion body can be made, it is very difficult to create a non-leaded glaze that melts and smooths over well and still has a low enough thermal expansion not to craze (super low expansion frits are needed).
While a variety of measures can be taken to make normal stoneware more resistant to thermal shock failure (e.g. more even cross section, thinner walls, smoother contours, better-fitted glazes, lower quartz in bodies, fewer dried-in and fired-in stresses) you will never be able to make it withstand a direct and concentrated flame. Vitreous porcelains can be made using high lithia materials, formulating them requires specialized knowledge and lab equipment and making ware from them requires highly specialized forming and firing methods (well beyond the capability of smaller operations).
Articles in periodicals deal with the subject from time to time. However, be cautious when they do not explain why their glaze and body recipes are resistant to thermal shock cracking (creating a low expansion fired ceramic is a critical mix of materials, procedure, firing schedule and the shape of ware). Consider an example of how published recipes often lack logic: For glazes to have super-low thermal expansion they would need low KNaO content (that means very low feldspar), high SiO2 and Al2O3 and employ low expansion fluxes (like Li2O, MgO). Yet recipes presented are often the opposite of this, high in feldspar and low in silica and kaolin. How can that produce a low-expansion glass? The ones that do at least contain a low-expansion melter (like spodumene) often have so little clay that it would be impossible to suspend them in a slurry.
Low expansion bodies are likewise easy to visualize: They would need low quartz particle content (thus fireclay would be a no-no, or any material sourcing quartz particles that would not dissolve) and plenty of low expansion grog (kyanite, mullite) and low expansion fine-particled filler (e.g. pyrophyllite). Such bodies have exacting firing requirements to develop or maintain the low expansion structure that could not escape mention. Yet body recipes presented online or in literature are often naked, completely lacking in technical documentation (this is a very technical subject). They may be loaded with quartz-bearing materials and even employ high percentages of coarse fireclay grog (how will you work with that?). Many do crow-bar the expansion back down with spodumene, talc and pyrax but, again, do not explain the firing intricacies needed and why they break all the other recipe rules. For example, some claim to be cordierite but forget to mention that cordierite needs 1300C+ (beyond cone 10) to begin developing its low expansion crystals (thus any thermal shock capabilities are due to grog content or other factors, not to imaginary cordierite crystals). Even if such bodies were fired high enough to develop some low-expansion cordierite crystals, what good is that if they are impregnated by high-expansion grog and quartz particles? Cordierite is also available as a powder (although not easy to get, check with Ferro). It is prefired, ware can be formed by plasticizing it with clay and/or binder additions and fire bonded it at lower temperatures (e.g. with a frit). Of course, these would not have the same thermal shock resistance as the pure material matrix.
If you are planning to make flame or ovenware you may be advised to consult the ASTM and other websites for testing procedures and services. The ASTM website has information on standards for ceramics and glass testing, listing a dizzying array of test procedures (and charging $39 for a PDF for each one). However, on closer examination, only a couple of tests apply to this. But be careful, they may send you back a test report with numbers that will mean nothing to you (you need to know those same numbers in the context of a wide variety of other ceramic types). Many of these tests must be done over a period of years and at multiple temperatures above and below the actual firing temperature. Technicians relate a long history of fired results to their history of testing to see the stability of their process. One test by itself without that content is often next to useless (for example, it could be that firing your ware 10 degrees higher could lose the low expansion properties, that fragility-of-process is very bad).
A recommended flameware recipe from a respected website (equal parts of 35 mesh grog, talc and ball clay). It looks good on paper but mix it up for a surprise! The texture is ridiculously coarse. Recipes like this often employ fire clays and ball clays, but these have high quartz contents. In flame test like this a ball clay vessel could easily fail in 5 seconds. But this one is surviving still at the 90-second mark. Or is it? While porcelain pieces fail with a spectacular pop of flying shards, these open-porous bodies fail quietly (note the crack coming up to the rim from the flame). There was a potential to create cordierite crystals (the reason for the talc), but what potter can fire to cone 14 to make that happen? But the porosity of 12.5% would be difficult to deal with. On the positive side, you could likely continue using this vessel despite the crack, the coarse texture would make the crack seem like a minor inconvenience. The previous being said, heating here is asymmetrical, creating exponential stresses from hot to cold face. Ceramic can absorb differentials if the heat source were more evenly distributed radially from the bottom distributing centre-outward symmetrically.
This is a flameware, made from a recipe promoted by a popular website. Are they serious? How could you throw this? Maybe it is possible, but we need an explanation. How could the page fail to mention how coarse this surface would be? How porous and weak ware would be? There are just so many body and glaze recipes taking screen space online, promising to be so much more than they actually area in real life!
The body is a 50:50 talc:ball clay mix, it is very smooth and slick so the only particulate is from the grog. In this case the grog addition is being used to make the body resistant to thermal shock failure (for use as a flameware). The body itself is not low expansion nor are the grogged particles. But the sheer quantity of aggregate particles and their size creates an open porous matrix that makes it difficult for cracks to propagate. Of course lots of burnishing, an engobe or a thick glaze layer will be needed to make this surface functional. We could call this the "crow-bar" approach to flameware.
It can if the flame is not too big! This is a roadside stand in Mexico in 2016. Each of these "cazuelas" (casseroles) have a small flame under them to keep the food inside warm. The pedestal is unglazed. The ware is thick and heavy. The casseroles are hand decorated with under-glaze slip colors with a very thin layer of lead glaze painted over (producing a terra sigilatta type appearance, but with brush stroke texture). These have been made and used here for hundreds of years. How can they not crack over an open flame? The flame is small and it is applied in the center of the bottom so that stresses are distributed radially and symmetrically. The ware is fired at what potters in Canada or the US would consider bisque temperature. It is porous, open and able to absorb the stresses. Some of them do have small cracks, but these seem to relieve stresses and prevent more. They know these pieces are not strong, so they treat them with care in heating, handling and washing. They know the glaze will leach lead or even dissolve if they put acidic liquids in them - so they don't do that.
This terra cotta cup (center) is glazed with G2931G clear glaze (Ulexite based) and fired at cone 03. It survives 30 seconds under direct flame against the sidewall and turns red-hot before a fracture occurs (the unglazed one also survived 30 seconds, it only cracked, it did not fracture). The porcelain mug (Plainsman M370) is glazed with G2926B clear, it survived 15 seconds (even though it is much thinner). The porcelain is much more dense and durable, but the porous nature of the earthenware clearly withstands thermal shock much better. It is actually surprisingly durable.
Ovenware clay bodies have a low expansion by virtue of materials in their recipe and/or the way they are fired. But potters bend the rules.
In the ceramic industry, cordierite is a man-made refractory crystalline material having extremely low thermal expansion.
Co-efficient of Thermal Expansion
Ceramics are brittle and many types will crack if subjected to sudden heating or cooling. Some do not. Why? Differences in their co-efficients of thermal expansion.
Thermal Shock Failure
A simple test any potter can do by making and firing square tiles and using a plumbing torch to see how long before they fracture.
|Oxides||KNaO - Potassium/Sodium Oxides|
Quartz is the most abundant mineral on earth, it is the main crystalline mineral form of silica (SiO
ASTM C554 - Thermal Shock Test Method for Crazing Resistance
Donabe Flameware Wikipedia page
Flameware article by by Ron Propst in Studio Potter magazine
Emile Henry ceramic ovenware from France. They claim products are "direct freezer-to-oven", they have extraordinary thermal shock resistance and heat retention properties. Products are glazed. It seems impossible but they are doing it.
|By Tony Hansen|
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