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Technical Tips Blog
The high porosity of this clay enables sealing against water leakage
This body has high porosity, almost 25%. It is L4410P, a dolomite-based low-fire whiteware, Plainsman Clays makes this as a product named "Snow". But this high porosity has some advantages, one of them is that it soaks up silicone sealer very well (called "liquid quartz" by some). The slip-cast piece on the left was sealed (you can see the surface sheen) and it is impermeable to water penetration (the glaze is not crazed so water cannot penetrate there either). The piece on the right soaks up water readily (on the lower unglazed portion). Sealing this specific body is doubly important because the dolomite particles within can rehydrate over time, especially in damp climates, causing pieces to crack. Even the foot rings of functional pieces should be sealed, not just to prevent hydration but also waterlogging.
Context: Why this dolomite body bisqueware is splitting after sitting around, Terra Sigilatta surface on a dolomite white earthenware, Stilts not always needed when firing pieces with glazed bottoms, ModPodge clay sealer, Clay body does not hold water
Sunday 19th March 2023
Same clay, same glaze, both drop-and-hold fired to cone 6. Why different?
This glaze is G3933A, the body is Plainsman M390. The one on the right is more matte and has a richer brown color, definitely a nice surface. The left one was fired using the PLC6DS drop-and-hold firing schedule. But the one on the right used the C6DHSC firing schedule, that one adds a slow cool down to 1400F. That gave the iron in the body time to bleed up through the glaze. And it gave the MgO time to do what it does best: Create a pleasant matte surface.
Thursday 16th March 2023
Bad and good glaze application: The difference was the rheology.
This is GR6-L, is the standard GR6-A Ravenscrag Slip cone 6 base recipe + 10% chrome tin stain (the body is Midstone, the inside glaze is G2926B, the firing schedule is C6DHSC). Chrome tin stains are picky about their host glaze, if it does not have a compatible chemistry they fire grey. Obviously, there is a love affair going on here! But the mug on the left has an issue. The glaze on the left has gone on in varying thicknesses and these are producing crystallizations and runs and the incising is not being highlighted. The one on the right is under control. What is the difference? The rheology of the slurry for the bad mug was wrong - the specific gravity was too high (the water content was too low). Even on a quick dip it was building thickness unevenly and way too fast. And there were drips that were so big they had to be shaved off with a knife! After the addition of a lot of water, to take the specific gravity from 1.55 to 1.45 it was watery enough to accept some Epsom salts to make it thixotropic. The difference was amazing, it went on totally smooth without a single drip, producing the result on the right.
Context: Formulating a Clear Glaze Compatible with Chrome-Tin Stains, Stains that work better in some glazes and not others, Thixotropy, Rheology
Tuesday 14th March 2023
Same body, glaze, thickness and temperature. Why did the front one foam up?
The glaze is G3933A, the clay body in Plainsman Coffe Clay. The front tile was fired using the C6PLST schedule that just goes to cone 6, holds for a short time and then free falls. The body is stained with raw umber, that material has a high LOI and gases right around cone 6. During the short hold at cone 6 the glaze percolated and foamed up with bubbles. The shut-off froze that in place. The mug was fired using the C6DHSC drop-and-hold and slow-cool-firing. The drop-and-hold shut off the gas expulsion the umber and gave the glaze a chance to shed the bubbles. The slow cool gave it lots more time to smooth out and heal every single pinhole.
Context: Drop-and-Soak Firing
Tuesday 14th March 2023
It dry shrinks much more yet cracks less. How is that possible?
Two mugs have dried. The local terra cotta native clay on the left shrinks 7.5% on drying, the porcelain one on the right only 6% (it is made using Kentucky ball clay). Yet few pieces of the terra cotta are ever lost due to drying cracks, even if it is uneven! For example, in a batch of a dozen mugs none of these will be lost whereas one or two of the white ones will always crack. Why? Dry strength. The clay on the left is very strong in the dry state, likely double or triple the white clay (the strength is a by product of its high plasticity and particle size distribution profile). That strength is enough to more than counter the extra shrinkage.
Context: One way to avoid drying cracks on handle-joins of engobed mugs, A step to prevent cracking at handle-joins on thrown mugs, One reason why stoneware clays are more convenient, How a kaolin and ball clay compare in a dry performance test, Drying Performance, Drying Crack, Green Strength, Clay Cracking During Drying
Monday 6th March 2023
Same body, same glaze, same firing. Why did one crawl?
The body: M370. Glaze: G2934Y (with added green stain). Firing: Cone 6 drop-and-hold. Glazing method: dipping (using tongs). Thickness: The same. Surface: Clean on both. The difference: Wall thickness. The one on the right was cast much thinner so the glaze took a lot longer to dry. Common pottery glazes contain clays which need to shrink somewhat during drying. The bond with the bisque, although fragile, is normally enough to prevent cracking during drying. But drying needs to occur quickly. Quick drying is only possible when the body has enough porosity to absorb all the water quickly. Otherwise, cracks appear and these become crawls during firing. A complicating factor is that stain and/or zircon additions make an already-crawl-susceptible glaze even worse. One or a combination of the following can be done to minimize crawling on even very thin-walled pieces: -Apply a thinner glaze layer. -Heat the bisque before dipping. -Glaze the inside and outside separately (with drying between). -Deflocculate the glaze to reduce water content. -Brush or spray it on in multiple coats.
Context: G2934Y, ChatGPT is surprisingly wrong about the causes of glaze crawling., Crawling, Glaze Crawling
Saturday 4th March 2023
How to stop low fire clays from waterlogging
Being fired at cone 04, this talc body is quite porous. Water is entering through the unglazed base. During an overnight immersion it penetrated upward to about 1 cm from the rim (and even travelled two-thirds of the way up the handle). So, is this clay and temperature practical for functional ware? Yes. The base can be glazed or siliconed, completely stopping water entry. Heating this in the microwave for an extended period did not fracture it. And even though the mug got incredibly hot the G3879 glaze did not craze - that gives reasonable assurance it will hold up over time. Low-fire bodies have plenty of advantages and they are certainly practical for functional use. Additionally, handmade items deserve common sense care during use (e.g. not leaving pieces in water for extended periods, even hand washing).
Context: G3879, Stilts not always needed when firing pieces with glazed bottoms, Clay Body Porosity, Microwave Safe
Thursday 2nd March 2023
Get a kitchen blender for mixing ceramic slurries
Blender mixing is invaluable in slurry preparation in small scale ceramics and testing. It is quick and so effective that not only are particle surfaces wetted much better but clay particles can actually be reduced in size (literally ground finer). Slurry rheology is also stabilized. For example, thixotropy can often be achieved naturally, without any additives. Materials that are otherwise impossible to mix into a slurry (e.g. bentonite, Veegum, CMC gum) are no problem. Even slurry-processed porcelains benefit, not only being more plastic, but firing to a more homogeneous surface and to greater density. Just visually, it is easy to see how much improved this MNP slurry is (a local clay with porcelaneous properties). It was mixed using our propeller mixer and seemed OK (on the left). But the improvement after only 20 seconds in the blender (right) is amazing.
Context: Testing your own native clays is easier than you think, Incredible Mother Nature’s porcelain, Blender Mixing, Slurry Up
Monday 27th February 2023
Medium fire white bodies used at cone 04. Is that smart?
Low-temperature white talc casting bodies have always been porous, 10-12% absorption is typical - pieces are basically just bisque-fired. People just pretend everything is fine! A stoneware potter might be horrified by this - until he/she sees the bright colors and brilliant glazes! If a dolomite body alternative is not available to you or your supplier is making a talc body that is no longer white there is an alternative: Use a middle-temperature white burning body. Why is this even possible? Their porosity at cone 05 is also 10-12% - but won't commercial glazes craze? Yes. So just make your own. This is G1916QL1 applied thickly on Polar Ice, P300, M370 (and L213 talc body), these are all plastic bodies but they can be adjusted for casting (removal of the bentonite). None crazed out of the kiln or after a BWIW test. I did this again with a thinner glaze application and at cone 03 (100°F hotter) and the results were even better. G1916QL1 can be mixed as a dipping or brushing glaze. But how can you guarantee no crazing? Fire even hotter, perhaps cone 02. Lower the thermal expansion further (as per instructions on its web page). And increase the silica in the body recipe at the expense of kaolin or ball clay (compensate with whatever bentonite is needed to get good casting properties). For a good cone 6 porcelain recipe for casting try L3778G. All of this can also be done with a plastic body, for that try L3778D.
Context: High thermal expansion talc body cannot be COE-calculated, Artware
Friday 24th February 2023
Stilts not always needed when firing pieces with glazed bottoms
The bases of many artware pieces can actually be glazed and then fired on the kiln shelf without using stilts. How? A thin layer of silica sand and a super thin layer of clear glaze on the bottom - just thick enough for the melt to soak in a little and seal the body against water penetration (of course the side walls are the regular three coats). How thick? Just experiment. In this example, I watered down some Spectrum 700 clear and applied one quick coat. It does not seem like enough but it produces a glassy surface that picks up very few grains of silica sand during firing. In the worst case, when applied too thickly, some grains of sand will stick, but even then they can be rubbed off and the piece is still ok.
Context: How to stop low fire clays from waterlogging, The high porosity of this clay enables sealing against water leakage, Artware
Thursday 23rd February 2023
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