Clay turns into ceramic at 570°
Clay is made out of three main ingredients.
Silica – glass agent
Alumina – bonding agent
Flux – controls temperature
Bisque firing is at 1000°
Vitrification (noun) is the process of melting that clays and glazes go through as they are fired to maturity. In a fully matured clay body, the spaces between refractory particles are completely filled up with glass, fusing the particles together and making the clay body impervious to water
Primitive clay (raku) has a high amount of grog (pieces of fired clay) which withstands refractory shock better.
Low fire clay is collected from low geological levels, it’s fired at low temperature as it’s travelled further and contains more flux. High fire clay is from high geological levels, it’s fired at high temperature as it’s travelled less and purer.
Earthenware – terracotta
Lot of flux, glaze firing around 1140° – 1170°, it can warp if fired any higher. Has low shrinking rate.
Not best for tableware as it can be porous but it’s ok with food safe glaze (stoneware better for dinner wear)
Stoneware – St Thomas, Crank, Buff
High fire clay, more refined material, 1220° – 1280° vitrification rate.
Casting slip is high earthenware.
Co-efficiency expansion – glaze and clay need same shrinking rate.
Porcelain has 20% shrinking rate
Bone china – casting slip 1220° – 1240°
Bone china is most translucent at 1240°
Porcelain – 1320° (1280° uni)
Plastic – leather hard – greenware (dry for firing)
Glassware (after it’s been fired)
Slip – casting (powdered clay suspended in water to pour. Deflocculant is a source of ions that charge clay particles to repel each other electrostatically and thus produce a slip that is thinner than it would otherwise be, agitate standing slip for 10-15min)
In ceramics, deflocculants are materials that you add to your slips and glazes to make them smoother, flow a little better, and even apply a little better. How, exactly, do they work though?
Sometimes when a dry, powdery material is placed in water, it dissolves evenly, creating a homogeneous solution. Other chemicals, however, will not dissolve completely, but may form clumps or settle to the bottom. In ceramics, and especially in slip casting, clumps and non-homogenous solutions can be problematic. If you are pouring liquid slip into a mold, you want the mixure to be uniform and to flow well. Deflocculants allow this to happen by decreasing the positive electrical charge that can occur between particles that become attracted, or ‘flock’, to one another. The addition of a deflocculant to a medium, such as casting slip, evenly suspends the particles and causes a more fluid flow.
If increased flow is the goal, why not just add more water? Why use a deflocculant? The advantage of using a deflocculant over water is that the addition of water will alter the shrinkage rate of your slip, or the application properties of your glaze. The more water, the higher the shrinkage rate of your casting slip, and high shrinkage is typically not so desirable. The more water, the thinner your glaze application will be, and that may not be so desirable, either.
Slip most often has a specific gravity of 1.75 – 1.8. To measure the deflocculant, you can measure a pint of slip.
Here, you weigh a known volume and divide the weight by that volume. For example, if 10mm weighs 17 grams, then 17/10=1.7 or record the weight of the water (100 gram mark) then record the weight of the slip to the same level and divide the weight of the slip by the weight of the water.
Decorative slip – usually 70% china clay to increase brightness in the colour and 30% ball clay to add flexibility for easier application. 10% can be added as well to make more durable and glossy.
Paper clay – Cellulain
Cellulain is a translucent white porcelain paper clay which stands out for its high
plasticity, what makes it exceptional for modelling. Acts like a capillary for water and air to go through clay.It is especially designed for creating impossible shapes with porcelain. The presence of paper fibres inside avoid formation of cracks and deformations problems. Its formula is very stable and provides a secure firing range between 1230 and 1270ºC. Toilet paper has a very high rate of Cellulain.
Cones are made from glaze material and measures the melting temperature in the kiln. It’s best to use a cone slightly lower than desired temperature.
Atmospheres – Reduction and oxidisation
Oxidisation is usually created in an electric kiln when heat source comes through the kiln elements.
Reduction is usually created in a gas kiln when supply of oxygen is shut down (flu)
Soda firing is an atmospheric firing technique where “soda” is introduced into the kiln near top temperature (2350°, ∆10). The soda vaporizes and is carried on the flame throughout the kiln. The soda vapors create a glaze when it lands on a piece (or a kiln post, or the wall of the kiln). Wherever the flame travels- so does the soda. When placing the pieces in the kiln during loading, you have to think carefully about when and where you want a piece to get lots of soda, or when and where you want a piece to be more protected. The kiln must be evenly loaded because the flame will travel on the path of least resistance (and therefore the soda will also be traveling on the path of least resistance). You also have to think about whether or not the piece is glazed. The soda is basically a glaze, and when two glazes mix, they can react chemically with one another and run down the side of the piece. Soda firings usually take 8-10 hours.
Wood firings can take 3-5 days to fire. Ash from the wood turns into glaze as it travels through to the back of the kiln, because of this shells must be placed under the pots as they’re calcium and stops the glaze running on the shelves.
Earthenware process stages
- Add grog, paper, molochite, straw, combustibles, stain body/oxide. iron
- Hand building – pinch, slab, modelling, carving, coil, paddle
- Moulding – press mould, slump, hump
- Casting – slip casting, layer colours
- Jigger and Jolly – industrial
- Ram press – industry process for powder clay
Decorate processes when leather card
- Slip – paint, sponge, paper resist
- Sprig – small mould to create relief
- Roller – decorative pattern on surface
- Sgraffito – scratch surface
- Shellac – impervious to water
- Screen print
Decorating process for bisque
- Underglaze – finer application with brush (use velvet glaze for a matt strong colour)
- Engobes – engobes are formulated using fritted material, which reduces shrinkage, gives your work a depth of color and tend not to have as much movement on the pot when firing, so they’re the most effective way to color your pot if you are using intricate detailed designs and patterns, particularly if you’re using several different colors.
- Oxide wash – natural occurring mineral, hasn’t been processed so the outcome will be unpredictable.
- Stain and under-glaze – commercially processed, what you see is what you get
- Under-glaze pencil and crayons (stick to black and blue)
- Under-glaze pens
Glaze 1140° +
Decal 830° – 850°
Lustre 750° (pure metal compound on glaze)