Potters & Sculptors - Making Rock from Mud
FROM MY EXPERIENCE, IT IS BETTER TO USE A PORCELAIN CLAY. I HAVE HAD PINHOLES AND BLISTERING WITH A "B MIX TYPE OF CLAY, AND I THINK A STONEWARE WOULD MOST LIKELY PINHOLE, THE GLAZES WOULD BE LESS VIBRANT WITH STONEWARE.
I tried for years to use Laguna B-Mix and other white stoneware and pretty much had maybe 40%+ failure rates due to pinhole problems. I switched a while ago to Dave's Porcelain from Laguna and my failure rate dropped to maybe 10%. Still not perfect but greatly reduced pin hole problems. Now with the 10% failure rate I embrace imperfection and still sell pieces with a pin hole here and there. Did a show last week and sold pieces with a couple pin holes here and there. The pieces were so beautiful despite the pin holes that it didn't matter. FYI!
P.S. - I do use Steven Hills spraying and layering techniques and use the SCM glazes and Hannah's Fake Ash Glazes but outside of that I pretty much use other glazes that I've tested and customize and mix myself. Even Steven Hill himself recommends the use of Porcelain over stonewares. I think he recommends the Tom Coleman porcelain but other porcelains will work. Less impurities in the clay and less outgassing.
There's a number of Laguna Clays, Cone 10 Half and Half is one, which begin to decompose at Cone 6 when fired with a lithium fluxed glaze which results in pinholes.
It's the lithium in the Strontium Crystal Magic which causes the pinholes. This is worse with a loose or open bisque which allows the soluble lithium carbonate to penetrate deeply into the bsique.
I have no explanation for the reaction, but tell our studio members to trial lithium fluxed glazes on test tiles made of the clay they've chosen. Changing to a non-lithium glaze is less disheartening than ending up hundreds of pinholes.
This was a discarded project which became a test tile, revealing pinholes from clay decomposition with a lithium glaze.
This sample of Weathered Bronze Green made with Spodumene containing lithium show the same undesirable interaction with one of the problem clays.
Pinholes were my personal trial and tribulation as a student. My first year half of my work was ruined by it. I blamed everything. I would refire stuff multiple times trying to save a piece from pinholes - futility of course. I was sure it was this or that but then other students were not having the same problem.
Norm mentioned the incompatibilities between clay and glazes - he amazes me with how much he knows. But don't forget you can introduce organic contamination on your bisqued pieces. Some people only handle bisque with cotton gloves. Hand creams or moisturizers (common used after throwing) can really contaminate your piece. Sitting on the shelf in a dusty place can too.
I found more pinhole problems when I used glazes at school even though I was bringing the pieces home and firing them in my own kiln.I suspected the glaze buckets were being fouled by innumerable students dipping and pouring and stirring the buckets using moisturized and hand cream smeared paws (using the toilet brushes of course)..But most likely it was the way I was handling the bisque during glazing. Now hands get washed before handling the piece. Bisque is wiped before glazing. Glazed pieces are stored under visqueen before firing.
I have relatively few pinholes anymore and practically no craters unless intentionally created. My bisque program slows to a crawl between 1100-1400 and the spyholes stay out until their is no chemically bound moisture being released anymore - this is usually around 1380. Hold at Cone 04 peak temp 20-30 minutes depending on how full the kiln is and if there any thick pieces in the firing. Don't have a kiln vent but wish I did. Like Norm mentioned the kiln lid distorts at peak temps and provides a nice vent.
I would suggest not using stoneware , maybe a B mix kind of clay. I am playing around with a stoneware buff clay, and experiment with scm(cone 6-7) formula. I spray the scm very thin and using some glazes I have been experinenting with, and seems to work fine. it takes a lot of experimenting. I just bisque at slow bisque used with the e-zy programs from l and l kilns. Remember , when using a stoneware clay, your glazes with not be as vibrant compared to using a white clay body/porcelain.
I have been doing clay studies for the last few years. I finished with porcelain and have moved on to stoneware. The primary flux in stoneware in potassium, whereas porcelain relies mostly on sodium. In addition, stoneware clays usually have a certain % of naturally occurring potassium: one reason you do not see the level of flux additions as you do with porcelain. That said, I have also noticed in my studies that potassium off gases much more profusely than sodium in a cone 6 schedule. The usual fix in the pottery biz is to do extended holds, but that is not always the most efficient fix.
Stoneware bodies rely on mullite production to produce a remotely vitrified body. Due to clay particle sizes, flux levels, and formulation blending: 2% absorption is considered "vitrified" by all accounts. Mullite production actually starts around 2050F and reaches its peak around 2170F. If you are doing a fast ramp firing up to 2180 +/-, you are blowing through these critical temps. This results in trapping gases near the surface, as the interior of the body has already closed its porous matrix.
The yellow line is the edge of the test bar: above it is the clay face, below it is the interior body. Notice it looks like swiss cheese just under the face of the clay. This is trapped potassium gases; both sodium and potassium are in a gaseous state at 2230F. I would suggest you fast ramp to 2050F or so, then slow the ramp down to 130-150F to your peak. This will allow the extra time for mullite formation, and for off gassing of the potassium.
Thank you Tom,
I, personally,do not have pinholes when firing Stephen Hill's glazes on stoneware. I use Cone 6 Pottery Supply House (in Canada) stoneware #519 and I have fired using both Stephen Hill and Ron Roy schedule. However I have had this problem with other glazes and will now follow your instructions.
I had the good fortune to spend three days with Ron Roy back in March, prior to the NCECA conference in Kansas City. Nothing like two clay nerds prating on for hours about formulation and firing. If I recall, Ron stated a peak of 2190? with a 30 minute hold in his book? We have since been emailing back and forth over PSD (particle size distribution) and flux levels in porcelain and stoneware bodies. Both bodies form a glassy matrix; but the mullite in stoneware requires a higher level of alumina to form properly.
I have been talking to the various clay mines in the States updating the data in my glaze calculator tables. There have been some changes in the last two years: both in iron levels and in naturally occurring potassium levels. Unlike kaolin in porcelain which have very little KnaO levels with the exception of one: most clays used in stoneware have a notable amount. These levels change as they dig through the various stratas of the mine. Although I cannot say with certainly, I am highly suspicious of these changes are playing a role in recent stoneware (pinhole) problems. Iron levels in two clays have increased by over 10%, so you will see that change also.
By stopping in the 2050F range, and slow ramping up to the 2180 +/_ range allows for more mullite formation, which thereby decreases absorption. This also allows for much more off gassing to occur before your glaze reaches its peak melt: thereby alleviating the problem of clay off passing being trapped in the molten glaze. However a word of caution: your hold time may need to be adjusted downward because the slow ramp will certainly increase heat work done.
The only time I look at glaze as being the problem child creating pinholes is when formula limits for KnaO are exceeded. The other is when extra thick applications of glaze are used to produce certain effects. Glaze recipes that have over 50% Nep Sy, Kona F4 (minspar) are automatically suspect for me. A 2 second glaze dip only amounts to 0.15-0.20 grams of glaze per square inch. In a typical application, off gassing of the glaze is rather minimal and short lived: always exceptions however.
Here is another example of pin holing:
The white glassy/glossy surface was in contact with the shelf. The upper part (left) did not fully mature. Notice the color is the same as the pin hole example above? In porcelain, the fluidity of the melt is very similar to glaze because it relies on the same basic fluxes as glaze does. It takes more time to move through the clay body uniformly. Porcelain is KnaO/silica/metakaolin. Stoneware is Al2O3/Silica- mullite. (spinel).