I have a profile drawing I want to match (upper left). On the left is my first attempt to tool a model. Even though I attempted to jigger the plaster to match, the profile is way off. I lost the enthusiasm to even get it smooth. For the one on the right I 3D-printed a shell, poured the plaster in and then smoothed off a bit on the wheel after the set. It matches the template and it is perfectly round (because I have a good 3D printer, a Prusa MK3S). This is revolutionary! That drawing: I hired someone on upwork.com to draw it for me using Fusion 360. He draws things in such a way that I can fine tune them.
Getting a 31 inch porcelain plate through drying and firing without cracks
What does it take? Three months! Porcelains are fine grained and, for heavy pieces, they will not tolerate uneven drying at any stage. These cone 10 plates are made by Peter Flanagan at Okanagan Pottery in Nelson, B.C. Firing is also a real challenge. Pottery porcelains are high in quartz, getting a piece like this down through quartz inversion (~1200-900F) without dunting is only possible if done very slowly. The fact that ancient Chinese potters made very large porcelain pieces means they knew about slow cooling also (and it was a natural consequence of the heavy kilns they used). But our modern kilns cool quickly so the drop must be slowed. Peter adds an extra level of "super humanness" by actually lustre firing these pieces, that means 2 more trips through the hazardous quartz inversion territory! If you do this be prepared to fire super slow (e.g. 25 degrees per hour) through this range.
For even coverage white majolica glazes must be applied by dipping
The mug on the left has three coats of Spectrum Majolica base, painted on by brush. Drying was required after doing the inside coats, so the total glazing time was several hours. The glaze layer is way too thin and it is not even at all! The one on the right was dipped in a 5 gallon bucket-full of Arbuckle white (that was weighed out according to a recipe and slurried at 1.62 specific gravity). It took seconds to dip-apply, the thickness coverage are good. As is obvious, it makes sense to make your own base white. Then, you can decorate using the overglaze colors (e.g. the Spectrum Majolica series). Another advantage of making your own white is that you can splurge on the amount of opacifier (in this case 9% zircon and 4% tin oxide), to achieve maximum whiteness and opacity. And, you can proportion a mix of two frits (having higher and lower thermal expansion) to fine-tune the fit with the body (a big issue at low fire).
By the magic of delflocculation, this powder will mix into that water and still fit in the container
This is 1100cc of water and 3000 grams of M370-2 casting. Amazingly, it is possible to get all that powder into that little bit of water. And still fit in the container (2250cc) and still produce a very fluid slurry for casting. How is this possible? That water has 11 grams of Darvan 7 deflocculant in it, it causes the clay particles to repel each other such that you can make a liquid with only little more water than is in a throwing clay! This is a test mix of M370-2 casting (it uses a large-particle kaolin), my pieces cast in 7 minutes (less than a third of the normal time). Using a good propeller mixer (in a bigger container of course) the slurry can be mixed silky smooth in a couple of minutes.
Does a grog addition improve thermal shock resistance?
Pyrax (Pyrophillite) is a mineral having a very low thermal expansion. It stands to reason that if we can maximize its percentage in a body and not fire the body to a point that changes the crystal structure, it will be resistant to thermal-shock-resistant cracking. To that end I mixed it with only kaolin (ball clay would add some quartz that would increase thermal expansion) and made slip-cast pieces. I fired them to cone 2 (after finding that by cone 4 shock-resistant properties begin to decline). As you can see from the video, the addition of grog actually harms the performance! The higher the Pyrax, the better. Will this work for kiln shelves? Yes!
Produces an appearance very similar to dolomite-matte-glazed ware fired in cone 10 reduction. The effect would be similar using speckled bodies made by other manufacturers. Pieces made by Tom Friedman.
A heavily grogged casting body still casts with a smooth surface!
20% 20-40 mesh grog was added to a Pyrax/Kaolin thermal shock body. While the insides of the pieces have a very rough surface, the outsides are smooth! Grogged casting slips have issues with the particle settling during storage and casting, however in this body the grog suspends long enough for a 15 minute casting time (and it easily mixes back in after storage). Pieces can be put into the kiln wet-out-of-the-mold and fast-heated to 250F and they do not crack.
Trafficked online recipes waiting for a victim to try them!
You found some recipes. Their photos looked great, you bought $500 of materials to try them, but none worked! Why? Consider these recipes. Many have 50+% feldspar/Cornwall/nepheline (with little dolomite or talc to counteract their high thermal expansion, they will craze). Many are high in Gerstley Borate (it will turn the slurry into a bucket of jelly, cause crawling). Others waste high percentages of expensive tin, lithium and cobalt in crappy base recipes. Metal carbonates in some encourage blistering. Some melt too much and run onto the kiln shelf. Some contain almost no clay (they will settle like a rock in the bucket). A better way? Find, or develop, fritted, stable base transparent glossy and matte base recipes that fit your body, have good slurry properties, resist leaching and cutlery marking. Identify the mechanisms (colorants, opacifiers and variegators) in a recipe you want to try and transplant these into your own base (or mix of bases). And use stains for color (instead of metal oxides).
G2926B with 10% Mason 6304 Stain on Polar Ice Casting
2% zircon also was also added, it helps prevent micro-bubbling. The PLC6DS (drop and soak) firing schedule was used. The G3806 base clear glaze is normally better than G2926B for really bright colors but this stain is an exception.
Supercharge the plasticity of cone 6 reclaimed clay
If your reclaim is short and non-plastic you can make it better-than-new by using an additive of 50% ball clay and 50% bentonite. While only a few percent bentonite supercharges the plasticity of any clay body it is almost impossible to get it to mix into a wet slurry or plastic clay. But thoroughly shaking it together with ball clay (in a plastic bag) separates the super-tiny particles of bentonite between the almost-as-tiny particles of ball clay, that new powder will easily mix with water. And it fires to a tan-buff stoneware at cone 6 so it won't change the fired appearance of most buff or brown cone 6 stoneware bodies. There is one downside: I can leave a scum on your plaster batt if your bentonite is high in soluble salts, so test on a small bat first. Or dewater by another method. Or use a dedicated batt whose surface you can scrape periodically.
BEWARE of leaving outsides of functional ware unglazed
This mug is made from the strongest porcelain I have, it is so vitreous that the bare fired surface does not even coffee-stain. So I glazed it only on the inside. That created a time-bomb waiting for hot coffee! Three others did exactly the same. Four other mugs glazed on the outside were fine. Why? Glazes need to have a lower thermal expansion than the body so they do not craze over time. When ware is glazed inside and the compressive forces the glaze finds itself under keep it crack free and also significantly strengthens the piece (like pre-stressed concrete). But here there is no outside glaze to be counteract the inside one pushing outward. When suddenly heated it pushes even harder. Structural weak points, outside surface imperfections or pronounced contour or thickness changes provide crack-initiation-points to relieve the stress. The only way to make this inside-only-glazed technique work is carefully tuning the thermal expansion of the inside glaze. That means a lot of testing and a lot of broken pieces.
What is the temperature difference between these two cones?
Four degrees F. I was consistently getting the cone on the left using a custom programmed firing schedule to 2204F. However Orton recommends that the tip of the self supporting cone should be even with the top of the base, not the bottom. So I changed the temperature to 2200F and got the cone on the right.
In 2015 we did a project comparing common cone 6 fluid-melt base glazes, picked a favourite (Panama Blue) and fixed it's slurry issues and crazing. Fluid-melts almost run off ware when applied thick, but they host stains & opacifiers to produce super-gloss, super-brights. That recipe, G3806C, has been among the most popular pages on our site. In 2019 we moved the thermal expansion much lower (from 7.3 to an incredible 5.7, it can survive a 325F-to-icewater test on our toughest-to-fit porcelain). The best variation, G3806N, is fluidity-controllable (by adjusting kaolin content), durable (double the Al2O3/SiO2 of other common ones) and employs lithium, strontium and magnesia frits. This version, G3806E, has 4% added copper oxide and sources SrO, Li2O and MgO from strontium carbonate, spodumene and Frit 3249. Follow the link here to see the entire history of this development effort (beware, there are multiple pages, each with many columns).
This cone 6 black glaze looks glossy until placed beside the cone 04 one
The cone 6 one is on the left, it contains about 25% frit. Both are colored using a black stain. That low fire glaze on the right has a high percentage of frit, likely more than 80%, that is the main reason for the beautiful surface. Frits are really fantastic, and standard practice in industry. However potters have been slow to adopt them, thinking they are more expensive. But from a "total cost" viewpoint, they are cheaper.
This was left for 24 hours. Wrapped in stretch wrap. Then the surface of the glaze was inspected under a lamp to detect any differences between the lemoned and non-lemoned surfaces. Lemons are highly acidic. This glaze passed because the base recipe, G3806N, was methodically developed so that it has plenty of Al2O3 and SiO2 (in the fired chemistry) to build a stable glass.
Our base glazes plus opacifiers on a dark burning body at cone 6
The body is Plainsman M390. These are commonly used base glazes. The top one is an MgO matte, next down is a calcium matte. They behave very differently to these additions. Notice also the difference when titanium dioxide is applied thickly. Tin oxide fires whiter than zircon (e.g. Zircopax). Each opacifier has issues. Tin is expensive. Titanium is difficult to mix into the slurry (screening required), not as white or opaque, variations in thickness produce more difference in results and it can turn blue. Zircon is more likely to cutlery mark, twice as much is required and it amplifies the color of any iron present.
It takes 80 pallets of dry materials to make a run of 4000 boxes of M370
M370 is now our second-most-popular clay body. Are you curious of the recipe? It is really quite simple. It has enough silica to resist crazing. It has enough Nepheline Syenite to mature to 1% porosity at cone 6. The rest is a mix of #6Tile kaolin and M23 ball clay with a little added bentonite to push up plasticity.
G3806C has been our recommended base recipe for reactive glazes (by the addition of colorants and opacifiers). It excels for copper blues, for example. But its thermal expansion is high enough that it crazes on some bodies (e.g. Plainsman P300). To adjust it (via glaze chemistry) I introduced some low expansion Li2O (from Spodumene) at the expense of high expansion KNaO, this dropped the calculated COE from 7.1 to 6.6. The melt fluidity, shown here at cone 6 (its most important feature), is exactly the same. The color is bluer. But not as dark, so copper oxide might be better. Or a higher percentage of copper carbonate. The base recipe (without the copper and tin) is potentially very valuable to create other reactive effects that depend on melt mobility. Why? Because it is very difficult to create a high gloss melt fluid glaze that also has a low thermal expansion.
It is impossible to dry this clay. Yet I did it. How?
These are made from a 50:50 mix of bentonite and ball clay! The drying shrinkage is 14%, more than double that of normal pottery clay. It should be impossible to dry them, the most bentonite bodies can normally tolerate is 5%. Yet notice that the handle joins with the walls are flawless, not even a hairline crack (but the base has cracked a little). Remember that the better the mixing and wedging, the smaller the piece, the thinner the walls, the better the joins, the more even the water content is throughout the piece during the entire drying cycle and the more damp of a climate you live in the better your drying success will be. What did it take to dry these: 1 month under cloth and plastic! I changed the cloth every couple of days. So by implementing these same principles you will have better drying success.