Glazes can be really pretty decorations, and if your ceramic piece has a lot of flaws in it the right glaze can take your attention away from the flaws and put it back on the glaze. There are some basic things you need to know about glazes. Glazing takes two steps:
- formulate base
- add colorant
Three Basic Components (chemical table)
- Glass former (XO2) X being a variable
- Flux (XO, X2O)
- Refractory (X2O3)
A glass former is the part of a glaze that is glassy, the most common glass former in a glaze is Kaolin, though Potash Feldspar also has glass former properties since most ingredients do more than one thing.
A flux is a chemical that is added to a glaze to lower the temperature at which it matures. Common fluxes are Potash Feldspar and Gerstley Borate.
A refractory is the part of the glaze that keeps things from melting too much, and keeps it all from dripping off your masterpiece. Most common refractory is Gerstley Borate.
A common Basic Glaze base would be:
- Four Parts Kaolin (Al2O3, 2SiO2, 2H2O)
- Three Parts Potash Feldspar (K2O, Al2O3, SiO2)
- Three Parts Gerstley Borate (2CaO, 3B2O3, 5 H2O)
Most chemicals fall under the chemical chart above, but there are some exceptions. One exception is that 3B2O3 is also a flux even though it's chemical composition lends itself to only a refractory.
To make an actual glaze with color is a little more complicated. A glaze recipe that we were given in class is this:
Delanie's Red/Brown
- F-4 soda Feldspar 20.2gm
- Silica 30.22gm
- EPK 3.96gm
- Gerstley Borate 31.89gm
- Talc 13.91gm
This is the recipe without the iron which gives it the color or the water. Once you have the dry ingredients mixed, add a couple of grams of iron, and then add water. This recipe makes 100 grams and can be doubled or quadrupled easily. The amount of water varies since water just suspends the chemicals so that they can be coated on the ceramic piece evenly, and since bisqued ceramic pieces absorb a lot of water it works out evenly. The water should be enough that it seems to dissolve when stirred (but please note that it will not actually dissolve since the chemicals cannot be water soluble since it would no longer be a glaze because the chemicals would be absorbed into the clay instead of being suspended topically). You also don’t want to add too much water since this will dilute the glaze.
When creating a glaze, you will want to do a whole series of test tiles, make sure to number them and mark each subtle change you make on paper. This technique is called a line blend, and it's where you take a glaze and add something slowly to it, note this doesn’t form new glazes, it just adjusts one you like.
some Terminology:
- Feldspar-Potash, cone f4 feldspar is not changeable with soda ash.
- Craze-Crackling when you want it, crazing when you didn't plan on it, it's when the glaze is too small for the clay and it has to crack to try to sit evenly, This is a defect
- Shivering-This is the opposite of crazing, this is when the glaze is too heavy, too big, and it pops off. This is also a defect
- Coefficient of Expansion-When things expand and contract together, the higher the coefficient the more crazing you might/will get.
- Frit-Companies take chemicals that are normally water soluble, chemicals like lead and boran that are hazardous to our health if ingested and they fire them with silica and grind it to a powder, this process makes the chemicals safer to use since they are no longer water soluble and will not leech out of the glaze after firing.
- Fusion Point-The point when the glaze melts.
- Vitreous-When it's solid, the maturing point of glaze. The optimum density.
- Interface-Where the glaze and the clay body meet, where they fuse.
- Oxide-Naturally occurring colorants:
- Stain-Commercially made oxides that you can use straight, they are dependable, but they are also expensive.
- Glaze-Glassy coat that can add color and strength to the clay and also makes it non-porous
- Refractory-Heat resistant, keeps things from flowing.
- Flux-Lowers the melting point
- Volitalization-When certain chemicals go airborne, this especially happens in reduction kilns and raku fires; copper is one of the most volatile. To stop this from happening don’t fire copper glazes with sensitive (white esp.) glazes.
- Opacifier-Something that makes the glaze opaque, a chemical like zinc.
- Bentonite-Gummy, it's a suspension agent, you add it to your liquid glazes to keep the chemicals from settling. Interestingly this is the main ingredient in cat litter, and you should be able to get it at health food stores for cheap.
- Eutectic-Singly the chemicals will not melt, but together it lowers the melting temperature.
Ceramics is fun, and I would suggest anyone take a class in it at their school, it can also be done at home as an expensive but rewarding hobby.
All info gleaned from my ceramics class in school. For more info look online.