This imposes a flame on the center of a tile and reports the amount of time until it cracks and the fired size. If rolled from plastic clay, the tile is made to be 3 inches square and 3/16 thick.
A 3 inch by 3/16 thick tile is flamed and timed until it cracks. The crack usually is audible and sometimes spectacular (the tile flying apart). However if the body is heavily grogged the crack can grow slowly (often from the edge inward). We log the result using the TSFL test.
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 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.
The porcelain on the left is highly vitreous, it cracked in 12 seconds, but the other lasted until 25! Why? Only theories, both of these should be failing sooner. The vitreous nature of the one on the left could be giving it the sheer strength to endure longer. The one on the right is Crystal Ice (from Plainsman Clays). It has very high quartz, which should have accelerated the failure. However it is not completely vitreous, the voids within the matrix could be giving it resilience. Another factor could be that the glaze is under more compression, adding strength. This whole thing is counter intuitive, porous bodies like earthenware normally endure best and vitreous porcelains the worst. Yet this same test on my earthenwares cracks them in less than ten seconds?
SECS - Seconds until failure (V)
MODE - Manner of failure (V)
Silent (crack appears), Ping (audible), fracture (tile flies apart).MM - Thickness of tile - mm (V)
CM - Width of Square Tile - cm (V)
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.
Flameware is ceramic that can withstand sudden temperature changes without cracking. The low thermal expansion of true flameware makes craze-free glazes very difficult.
When sudden changes in temperature cause dimensional changes ceramics often fail because of their brittle nature. Yet some ceramics are highly resistant.
Liner-glazing is a very good way to assure that your ware has a durable and leach resistant surface. It also signals customers that you care about this.
|By Tony Hansen
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