Question:

How do I make my own glazes?

Balancing your glaze

Glazes need a balance of the 3 main ingredients: Silica, Alumina and Flux.
* Too much flux causes a glaze to run, and tends to create variable texture on the surface. The texture may vary from shiny, where the glass is balanced, to matt where the excessive flux oxides may form visible, possibly lumpy, crystals.
* Too much silica will create a stiff, white and densely opaque glass with an uneven surface. It will be glossy in spots, but the suspended silica can form crystals producing harsh dry surfaces. Too much silica will also inhibit the melting of a glaze, and the resulting surface will be roughly textured like sandpaper.
* Too much alumina causes a glaze to stiffen and tend towards opacity, again with a textured surface where it is dry in spots. Glazes will often have pinhole defects. Too much alumina can inhibit the melting of the glaze to the extent that a poor quality matt glaze results, one that looks matt but is prone to discoloration.

Basic glaze additions

Plus a glaze may include one or more additives:
1. Opacifiers – to make the glaze opaque instead of transparent. Examples: tin oxide, zirconium or Zircopax, titanium, zinc
2. Suspenders – to keep the glaze in suspension instead of settling out. Examples: bentonite
3. Colorants – to provide various colors. Examples: cobalt oxide, copper oxide.

What is a Flux?

To provide flux in the glaze, we need a material that contains one or more of the following:
Li2O=Lithium Oxide, comes from Lithium carbonate, Petalite, Spudomene
K2O=Potassium Oxide; comes from Potash Feldspar, frit
CaO=Calcium Oxide, comes from whiting, limestone, wollastonite (also provides SiO2), wood ash, bone ash, dolomite (also provides MgO)
MgO=Magnesium Oxide, comes from magnesium carbonate, dolomite (also provides CaO), talc
ZnO=Zinc Oxide, comes from zinc oxide
SrO=Strontium Oxide, comes from strontium carbonate
BaO=Barium Oxide, comes from barium carbonate
PbO=Lead Oxide (not used much due to toxicity)
Na2O=Sodium Oxide, comes from feldspar, FRIT, cryolite, nepheline syenite
TiO2=Titanium Dioxide, comes from pure titania, rutile
ZrO2=Zirconium Dioxide, comes from zirconium dioxide, zircopax, zirconium silicate
SnO2=Tin Oxide, comes from stannic oxide (SnO2 white), stannous oxide (SnO black)
B2O3=Boric Acid or Boron, comes from Colmanite, Gerstley Borate, CadyCal. Effective for lowering the melting point of a glaze.
If you've worked with glaze recipes at all, you probably recognize many of these terms, and can start to understand what they are used for.
You can take any glaze recipe, and break each ingredient down into its chemical composition as shown last week. An easy way to do this is by looking up the material in the DigitalFire database. DigitalFire database
Once you have the chemical composition of the ingredient, you can see what it contributes to the glaze. For example, is it primarily contributing silica, alumina, or a flux? Often a single ingredient contributes a combination of these. For example, Feldspar is primarily a combination of alumina and silica. And so is clay.

Understanding %'s

Glaze recipes are typically expressed by listing each raw material and its % by weight. The percentages add up to 100 Usually colorants and sometimes other additives (such as bentonite for suspension) are not included in the 100%, they are added on afterward.
Example:
Cone 6 Clear Base Glaze

Wallastonite 10%
FRIT 3134 30%
Kaolin 25%
Flint 15%
F-4 Feldspar 20%
Total 100%
Add 4% cobalt oxide for a deep blue
To mix this glaze, you take the total number of grams of dry material you are making, multiply by the % to get the grams of each material to add.
Example:
To make 1000g of glaze
Wallastonite = 10/100*1000=100 grams
FRIT 3134=30/100*1000=300 grams
Kaolin = 25/100*1000=250 grams
Flint = 15/100*1000 = 150 grams
F-4 Feldspar = 20/100*1000 = 200 grams
To double check, add up all the grams and make sure they equal 1000.
Then add 4/100*1000 = 40 grams cobalt oxide
This is as far as many people go. They make the glaze, test it, and often are unhappy with the results. We need to understand why!

Basics of a glaze

Every glaze is made of the following 3 materials:
1. Silica – Creates glass. Examples: quartz, flint, pure silica
2. Alumina – Stiffens the glaze so it doesn't slide off the clay. Examples: clay (kaolin, ball clay, or fire clay), alumina hydrate
3. Flux – Causes the glaze to melt at a low enough temperature to be used in ceramics. Examples: feldspar, whiting

Making a basic glaze

To make a glaze, we need to find sources of Silica, Alumina and a flux which are convenient to use, in a form that does not dissolve in water. Glaze materials can be broken down into their chemical compositions, and from there we can see what the effect of each material will be.
To provide silica in the glaze, we need a material that contains:
SiO2=Silicon Dioxide, comes from flint, quartz and pure silica.
To provide alumina in the glaze, we need a material that contains:
Al2O3=Aluminum Oxide, comes from feldspar, cryolite, clay.
FLUXES
Silica and alumina would create a glaze if fired hot enough. However, ceramic kilns do not reach the temperatures required. Therefore, we need to add fluxes, which lower the melting point.

Make your own

Make your own: This is the most advanced form of glazing. Using recipes, you buy raw materials and mix them. In addition to the other things, you will need recipes, which may be obtained from many books and web site. You also need the chemical, which make up your glaze, a scale, a sieve, and a temprement which allows experimentation. Sometimes your glazes won't turn out quite right. You will have to learn how to modify those glazes to solve whatever problem you're having. Other times they will be stunning.