Friday, April 15, 2016
Tuesday, April 12, 2016
Firing Clay at the Right Temperature (from Big Ceramics Store)
It is important for functional ware that clay reach maturity. Maturity happens at a certain firing temperature. Commercial clay’s are actually a mixture of raw clays and other materials which are combined to create good working properties, and fire to a certain temperature. If a clay is rated ∆10 that is theoretically the temperature at which it becomes “vitrified”, or becomes almost glass like. The particles become densely packed, all the water is driven out, and at this point that clay is waterproof. Fire a ∆10 at ∆6 and it is not vitrified and you may have other problems.
Many people think that glaze provides waterproofing for clay…If relying on a glaze for waterproofness, you need a glaze with very good properties for waterproofness, and one which also exactly fits the clay body so they expand and contract at the same rate. If not, the glaze will crack (craze) and even if it is microscopic crazing you cannot see, your piece will no longer be waterproof, it may not be food safe, or microwave safe or it could not be durable.
So this is why stoneware is much more durable, because the waterproofness comes from the clay itself. If you fire a ∆ 10 clay to ∆ 6, it will be fine for sculptural or decorative work, but it will not hold up to daily use, microwaves, dishwashers, etc. because it has not matured (it is still too porous). Over fire a clay, and first it becomes brittle, then it starts to melt. So you want a clay that is made for the temperature you will be firing at. That is why we don’t put ∆6 clay in a kiln firing to ∆10.
Many manufacturers will give a firing range for a clay, for example cone 4-6. A small range is probably justifiable because of variations in the materials, and the inability to always achieve an exact cone level through a kiln due to hot or cold spots. However, some manufacturers take this to the extreme, marking a clay as cone 4-10. This is hogwash. There is a single, specific temperature at which that clay will become vitrified. It simply cannot happen over a large range. In this case I would bet it is closer to ∆ 10 than ∆ 4, because they wouldn’t want to risk their clay melting in your kiln.
How do you determine whether a clay is matured? Do a porosity test Take a piece of clay and fire it to temperature. Weigh it. Put it in boiling water for 5 minutes, and leave in the water as it cools. When cold, remove the piece and dry it off with a sponge. Weigh it again. The difference in weight divided by the original weight time 100 is the % porosity. An ideal clay would have 1-3% porosity. (Zero porosity, in addition to being practically impossible is also not as strong because it is more brittle.) If a clay is around 4-5% porosity, it is “waterproof” enough to be good for functional ware. In the above example of the clay with the large firing range, the cone 4-10 clay fired at cone 10 may have a 3% absorption, and at cone 4 may have a 13% absorption. So while technically it can be fired over that large range, it will not have the same properties when fired over that range. A ∆ 6 clay fired at ∆ 4 might be 6% absorption compared with 4% absorption, and either may be acceptable. Note that for a piece to be “microwave safe” the porosity should be very low. Otherwise, water which has leaked into the piece can quickly expand and cause it piece to crack.
Also, for oven ware use, the clay used should have low thermal expansion. This means that it doesn’t expand much as it gets warmer. If a clay expands a lot as it gets warmer, it will tend to expand more where it is hotter (closer to the elements) and less where it is colder (further from the elements), which creates stresses and can cause cracking. Some manufacturers list thermal expansion, or at least tell which clay bodies are good for oven ware because they have low thermal expansion.
We have had guild members fire cone 10 clay in cone 6 without any visible signs. However, we have also seen blistering and bubbles in the glazes, and the pottery not standup to or deteriorate more after microwaving and/or dishwashing.
A few other thoughts. According to Richard Zakin’s Electric Kiln Book, a similar temperature in reduction will mature clay slightly more than in oxidation. Also, manufacturers typically rate the clays on the high side, to avoid their clay destroying your kiln if your temperature is off slightly. So, for example, I have never had any problems firing ∆ 5 clay at ∆ 6 in oxidation. But always test! I haven’t tried them all! And there can be variation from batch to batch, and kiln to kiln. So in a way it’s nice to have at least a small margin for error.
Keep a notebook! No matter how much you think you will remember how thick you put a glaze on, whether you dipped, painted or sprayed it, how many layers of glaze & which glaze came first, was, or even what glaze you used to begin with, you will forget! And scraps of paper get lost too. So use a composition book, spiral notebook or a 3 ring binder. Write down everything you do, as you do it. Some of us add number to each piece of pottery so we can write down all the details of a particular piece. The numbers come in handy. I often look at a cup or bowl I’m using & think I know what I did, but I can check the number against my glaze book even a couple years later, & find out what that glaze was, what kind of firing & exactly what I did. But what you’ll learn is that you probably never write enough. The more you do it the better you’ll get at what to write down. Some folks also add photos of their pieces. These things will start paying off right away. Get a glaze book.
Monday, April 11, 2016
Some Issues that determine whether a piece is appropriate for dinnerware use (From Big Ceramic Store).
- Resistance to abrasion (does it scratch easily with silverware?). You can test this yourself. It is usually a problem more with matt glazes than shiny.
- Ability to handle acidic foods. To test for acid resistance, slightly squeeze a slice of lemon to get the juices flowing, and leave the whole slice on the glaze overnight. See if the color changes. Another way is to pour white vinegar into a piece and let is set for 3 days. Pour the vinegar into a clear glass and see if any colorant is visible or if the glaze on the piece itself has faded in contact with the vinegar. If it does, there is some leaching going on. Finally, there is lab testing, described later.
- Ability to withstand alkaline dishwashing detergents. To test this, mix 50 grams of soda ash in 1 liter of water in a stainless steel pan. Bring to boil. Reduce heat to a simmer. Place samples in the pan, cover, and simmer for 6 hours. Compare the color and surface gloss to a similar but untested sample. (This test came from Mastering Cone 6 Glazes).
- Ability to withstand thermal shock. This does not mean that you can place a ceramic pan over a flame, or directly into a hot oven. It is very difficult to make pieces that can go directly over flames, and not something an individual should attempt. Ceramic casseroles, etc. should be put into the oven at room temperature, and brought up to temperature slowly. However, your customers might not know this, and even if you tell them, they probably won't remember. To test for thermal shock, place a test pot in the freezer for several hours. Then submerge the piece in a pot of boiling water. (Alternately, put the pot in the sink and pour the boiling water into it). Repeat this 3 times, looking for minute crazing on the glaze. It is also a good idea to do example what a customer would do. Take a completed piece out of the refrigerator, and put it into an already heated oven. Make sure the piece does not crack.
Terra sigillata Primer (From the Big Ceramic Store; See some of Russel Fouts' terra sigillata work at http://www.mypots.com
Last week someone was asking me about terra sigilatta and using it in Raku. So here’s some info on Terra sigillata. Susan Mattson and Pat Appling have done this successfully and they can probably answer any questions you may have:
Terra sigillata is a very smooth, lustrous coating of clay which resembles a glaze and is virtually waterproof.
These days, the name terra sigillata is used to refer to an especially fine coating of clay applied to a ceramic piece.
The silkiness and shine of terra sigilatta is due to the plate like shape of the clay particles and the use of only the smallest particles. Polishing this surface with your hand or a soft cloth lines up all the clay 'plates' and gives the surface its shine.
Most terra-sigillatas are made by a process of levigation in water which allows the larger particles to settle to the bottom, leaving the very finest, sub micron sized particles in suspension. These very fine particles are siphoned off and become the terra-sigillata.
The Method: there are many ways to make terra-sigillata, this is a simple method that works for me.
- Add dry or moist clay to a lot of water. The proportions by weight are usually 1 part clay to 2 parts water. This can be as high as 1 part clay to 4 parts water for a REALLY plastic clay, like a ball clay.
- Mix the clay and water very thoroughly to break down any lumps. Let the mix sit for a day and mix again. I do this over several days mixing for about 15 minutes a day. Mix thoroughly one more time and pour the mix into the largest, tallest, transparent containers you can find
- Add 7 grams of liquid sodium silicate per liter and mix thoroughly in the containers
- Leave this undisturbed for about 24 hours and it should settle into 2 or 3 visible layers. The layer you want should look like VERY thin milk.
- Siphon off the VERY thin, milky layer into another container.
- Congratulations, you now have terra-sigillata!
· You want to start with the most "naturally" plastic clays that you can. The amount of terra-sig that you get in the end will depend on the amount of very fine particles in the clay. Generally the more naturally plastic a clay is, the more terra-sig it will yield. I use the term "naturally" plastic because many clay bodies are only workable because they have had plasticizers, like bentonite, added. These clays probably won't yield enough fine particles to make your work worth the effort.
· In general, Porcelains will yield less terra-sig than "natural" earthenware clays.
· For White or "Colorable" Terra-sigillata, use a very plastic, white burning ball clay. Avoid low fire, white clays. Many of these contain "plasticizers". White terra-sigs can be colored using Mason stains or coloring oxides like cobalt. The stains or oxides should be as finely ground as possible otherwise the relatively large size of the oxide particles can interrupt the shine of the sub-micron size terra-sigillata clay particles.
· "Natural", low fire earthenware clays like Red Art make great TS. Most earthenware clays, through the natural process of their making are very plastic, have LOTS of extremely small particles and yield a lot of terra-sig. They also lend their own natural "earth" colors; reds, browns, oranges, yellows, etc. Color depends on the final burning color of the clay.
Want to do Minimal Wedging? Here is a diagram of how the clay comes out of the pugmill and is bagged from the bray. If you want to save time and energy in wedging, wedge it in the direction that the clay is already formed in. Instead of having to wedge +50 times you can wedge it quicker if you look at this diagram and continue the wedging in the direction it has already been done. This diagram is also posted at the wedging table. You should also use this same concept for our reclaim clay, Reclaim comes out as a cylinder with the clay in a spiral. So wedge this clay from the side and not end over end. You’ll save a lot of time. You can actually take our pugged clay, cut off a chunk and throw it on the wheel with the cylinder up without wedging, and start centering. However, it is always a good idea especially for beginners to wedge clay a little first just to insure that you don’t have any air pockets.
Drying tip (Troubleshooting for Potters, by Jacqui Atkin)Rims often get bumped and tend to warp. A great way to keep rims perfectly round as they dry is to place them over an upturned bowl. Cover the bowl loosely with a piece of soft plastic before inverting the form over it; this will allow for movement and shrinkage as it dries, otherwise the form may stick to the bowl & you’ll have cracks around the rim.