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FIRING
How fast can you refire bisque that was underfired?
Very fast.
If you tried to fire a bisque load and for whatever reason it underfired, you can refire it to bisque temperature. For example, my manual kiln shut off before it reached cone 06 due to some irregularities I was doing. The cone sitter dropped before the heat work was actually done.
Underfired bisque feels softer, more chalky, and not as pure white (for porcelain). In reality it probably reached at least 010-08, and possibly still usable, but I wanted to refire to 06 for proper glaze adhesion. As both physical water and chemically combined water had been fired out, and quartz inversion already happened, you could refire much faster as if you were skipping the slow process of firing the first time. I fired to twice as fast as I usually do, and all the pots came out fine.
For reference: normal bisque firing takes me 8 hours, refiring bisque took me about 4-5 hours.
How fast can you refire glazed pieces?
In my experience it will be the same as above. It was fine to fire quickly, but cool slowly or the same way as you would a regular glaze firing.
Will placing a pot too close to the elements cause warping?
Yes. This is only an issue for thin lipped or open forms, like cups and bowls. This is not an issue for forms that have a shape that turns inwards at the mouth (bottles, round vases, etc).
Leave at least 1-2inches of space between the object and the element. (This photo was taken with an iphone so it is distorted, there is at least 1.25" of space).
Protect sensitive things by placing them in the center on the kiln shelf, and handles facing inwards.
Does turning on a vent affect the kiln's firing or heating ability?
YES. From what I've read and heard, most people say it shouldn't or doesn't, but for me it did. Enough for me to see a big difference.
Turning on the vent means a longer firing. Fresh air is being pulled in, and heat is also able to escape (an electrical kiln). This is simple science. If you have a more sophisticated kiln that is more airtight, venting won't affect you as much, but I have an older kiln and it absolutely can add an extra hour of firing to reach temperature. Turning off the vent made the kiln much hotter and fire quickly, as I have tested this multiple times. (Of course you want to vent every time, to let fumes escape).
I have a vent that is basically a fan connected to duct that is held underneath the kiln. Three holes are drilled into the kiln lid and three drilled into the kiln floor. The vent draws air from these floor holes.
Do leaving peeps open or closing peeps affect the kiln's firing or heating ability?
Yes, same answer as above. It will allow fresh cool air in and heat escapes, leading to longer firing times.
If you do not have a vent, you may want to leave the kiln peeps open or even prop the lid by a tiny gap. This is for fumes to escape. Venting helps prolong kiln life.
Will a kiln work if my studio/space doesn't have a 220-240 volt line?
Jesus Christ, this was the one thing I wish I knew sooner. There are different kinds of elements rated for specific voltages, such as 240v elements, 208v elements, and 115v, 120v elements. The average NYC home has 208v. You need to get elements that can work with that level. 240v elements will not reach temperature because it is not receiving the electricity correctly. If you are in New York or nearby areas, you can contact Joe Catanzaro at jckilnservices@gmail.com for amazing service. @nykilnguy on Instagram.
GLAZING
Why are glaze buckets so gosh darn dirty?
It helps to wipe down the entire edge/mouth of the bucket after using a glaze. When you or students dip in the glaze, it may splatter and be messy. After every use, take a sponge to wipe down large messes on the inside of the lid and the entire mouth edge.
If glaze is left to dry on the sides of the bucket it gets chunky and cannot dissolve again without being sieved. If you train yourself and students to use a spatula or even a finger to wipe the glaze down the sides from Day 1, after EVERY use, you will never have messy dried up glaze crusting on the sides of the bucket. Clean bucket train goes woowoo! DORty bucket train goes boohoo :(
How much dry glaze material do I need to fit into a 2 gallon bucket?
6,000 grams. Recipes total up to 100, not including the colorants. Multiply 100x60, so each ingredient by 60. Not including water.
I am missing some but not all of a glaze material. What will happen?
If it wasn't a significant amount the recipe will most likely look similar to what you were aiming for. Of course it will vary upon WHICH material it is.
I was missing 20-30 grams of tin oxide in a 6,000gram batch of dry glaze material for a lavender purpleish pink cone 6 recipe. That's missing about 1/10th of an ingredient. The recipe is as follows for 6,000 GRAMS:
Custer feldspar: 1800g
Ferro Frit 3134: 1680g
Wollastonite: 1080g
Silica: 780g
EPK: 480g
Tin Oxide: 300g
Dolomite: 180g
Copper Carbonate: 45g
Chrome Oxide: 6g
I potentially added too much Chrome Oxide as well, because it was such a small amount and hard to measure accurately with the average scale. Still, the missing tin oxide and the overshooting chrome actually did not seem to affect the recipe much. So if you are a worried wombat you will probably be okay.
*By the way, this recipe is extremely runny when layering with other glazes, and although it can be a pain to apply it thick, the fired results looks the best when thick. Recipe posted by David Tsabar https://glazy.org/materials/19421
Can I use very old glaze? Or glaze that survived heat and cold?
Yes but the texture may be a little off during application.
Old powdered glaze, about 5-10 years old, was very bubbly: it bubbled in the bucket when mixed with water. I sprayed alcohol on the surface to break the bubbles. It also leaves a bubbly layer on the bisque, which can be rubbed away and will be fine.
Some matte or more opaque glazes may still be very bubbly looking after glaze firing. So just be aware of texture issues for POWDERED commercial glaze.
Old LIQUID commercial glaze, also many years old, that has been affected by freezing winters and swelting hundred degree summers, will have a disturbed texture. It will definitely need some mixing and sieving, and can be thinned with water. It may apply like a crawl glaze, flaky or gelatinous.
However, the fired results look perfectly normal.
Does a store bought kitchen sieve work for glazes? (as pictured on left)
No. The mesh size is too big for the strainer above with the strawberries. The best size is 80 mesh, pictured above right. It is made of stainless steel. It does not attach to a bucket so you can balance it over two sticks, or a wire rack.
What to do if glaze is too thin or has too much water?
Let it settle and siphon off the extra water at the top. Leaving the bucket without a lid on helps water evaporate as well.
SLIPCASTING
What to do if slip casting slip is too thin?
The commercial slip I purchased was too thin or watery. I poured the slip into large containers with no lids and let them air dry every day until they thickened up. It definitely helped.
Why does the stoneware slip feel different from the porcelain slip?
Stoneware slips that have a warmer and darker tone while wet probably contains ball clay and/or bentonite (pictured in the rectangular bucket). These material retains a lot of moisture because they have a small particle size but swells significantly and holds in water. Due to this small particle size, once that water evaporates out it will shrink immensely. It dries extremely slow as it is difficult for water to travel past the fine particles. It allowed me to cast really thin forms but took forever to dry.
For example, I was able to remove the casting from the mould in 1.5-2 hours with cone 6 porcelain in wintertime. With the white stoneware slip I had to wait 3-4 hours to remove it.
You should probably sieve the stoneware slip as well, because many times it will contain some contaminants or large particles.
There will be a slight difference when glazed fired. Even a grogless stoneware slip formula will feel grainier, like a stone, and more of a buff color.
Why are my slip cast forms cracking at the mouth?
It is drying faster by the mouth hole than the rest of the body. This can be resolved by covering the mouth hole. I used a little plastic lid and loosely placed it over the hole, as I didn't need it to be airtight. This did not significantly affect the rest of the body from drying, while keep the mouth wetter and more equalized.
The first cast is not releasing even though the plaster mould is completely dry?
This made me question reality at first, and it can be upsetting if you are rushing a project. So when the mould is too dry, it really sucks onto the casting as it draws moisture. The second and third casting will feel way better as the suction is not so severe. Sometimes you have to prepare to lose the first casting.
Why does my slip cast pieces have micro bubbles or bloating in the body after glaze firing?
It is probably overfired. Especially brands like Laguna, who sells clay and slip that is better suited for cone 5 or a cooler cone 6.
PLASTER AND WEDGING
Can you use Hydrostone to make a wedging slab instead of pottery plaster?
I chose to use Hydrostone for my large wedging slab because I wanted a material that was durable and more scratch resistant than USG No 1 Pottery Plaster. I don't like how sometimes plaster can disintegrate or get marred over time, risking contamination of the clay. I wanted something stronger: Hydrostone is the material used in Hydro-bats. It is 8-10 times stronger than regular plaster, and contains cement and crystalline silica.
I purchased it for $35.50 for a 50lb bag, not including shipping, in 2023.
Hydrostone has its good and bad side. The good news is that it is much more durable that pottery plaster, and thus less scratching, crumbling, and pitting on the surface. It is fine for wedging. But it was far less absorbent than regular plaster, so reclaiming took longer, so I don't recommend this to studios who are using slabs to dry clay.
The hydrostone must be drill mixed and poured quickly, and check the water ratios as it is different from regular pottery plaster.
What is the best way to wedge clay?
Ram's head wedging for sizes of clay that you can comfortable hold two hands around.
Spiral wedging or slam wedging for anything large.
I will always slam and cut wedge my reclaim first, and then spiral after.
Slamming compresses the clay and cutting it over and over ensures homogeneity as the layers multiply. Also good to check and remove contaminants or hard lumps as you cut it open.
LUSTER AND PRECIOUS METAL OVERGLAZES
Can I fire mother of pearl and gold luster at the same time?
If the temperature ratings are relatively close, yes. Even on the same piece. For example, Duncan, now Mayco brand, has gold luster rated for 019-018, while mother of pearl is 020. I fire them both in the same kiln to 019 with no issue.
If mother of pearl luster touches gold luster it can contaminate the effect and look a little dark or purpleish brown.
What happens if your luster application is too thin?
A luster application should be thin as in the entire surface is an even color (for example, the gold luster is a red color). It looks like blood, but should not be so thick that it is drippy and dark. As long as the red is even, there will be gold. Imagine one good swipe of lip gloss (you know I'm for girlies)
Now if it is TOO THIN, it will not have the consistency of lip gloss, it will be totally transparent like chapstick. It can look streaky, and the gold will fire a mauvy purple instead.
LIDS
Does freezing really help remove a stuck lid?
Yes. If knocking it hard with a piece of wood doesn't do the trick, I will freeze it. I put the piece in the freezer for a few hours, or overnight, and ran only the lid part under lukewarm water and it easily came off. This will only work if you are confident that there was no other areas glazed inside that would fuse them together.