Understanding Pottery Glazes
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. So let’s go further and understand why.
Every glaze is made of the following 3 materials:
Silica – Creates glass. Examples: quartz, flint, pure silica
Alumina – Stiffens the glaze so it doesn’t slide off the clay. Examples: clay (kaolin, ball clay, or fire clay), alumina hydrate
Flux – Causes the glaze to melt at a low enough temperature to be used in ceramics. Examples: feldspar, whiting
Plus a glaze may include one or more additives:
Opacifiers – to make the glaze opaque instead of transparent. Examples: tin oxide, zirconium or Zircopax, titanium, zinc
Suspenders – to keep the glaze in suspension instead of settling out. Examples: bentonite
Colorants – to provide various colors. Examples: cobalt oxide, copper oxide
To make a glaze, we need to find sources of each of the above which are convenient to use, in a form that does not dissolve in water. As we saw in the last tip (#34), 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 than 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
Silica and alumina would create a glaze if fired hot enough. However, ceramic kilns are do not reach the temperatures required. Therefore, we need to add fluxes, which lower the melting point.
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.
Now don’t panic! This isn’t chemistry class, and you don’t have to memorize this list! But 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 it’s chemical composition as shown last week. An easy way to do this is by looking up the material in the DigitalFire database. http://www.ceramicsearch.com/material/
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.
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 matte 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 matte glaze results, one that looks matte but is prone to discoloration.
SO HOW DOES THIS HELP YOU?
By understanding what different materials do, you can adjust a glaze recipe to change its characteristics or fix its problems. For example, you can make a transparent glaze into a matt glaze. You can stop crawling, pinholing, or crazing. You can lower the melting temperature of a glaze. You can make a substitution if you run out of an ingredient. This ability completely changes the way you work with glazes.
UNITY FORMULA AND GLAZE CALCULATION PROGRAMS
You may have heard of something called a unity formula, or Seger formula. This is a way of expressing a glaze by the ratios of its oxides rather than % of raw materials. It is one of the primary methods used in analyzing glazes. I’m going to skip the detailed math. But the concept is that using information about each raw material, you create a ratio of the amount of flux to the amount of silica and alumina. These ratios can then be compared to ones which have been determined to work in a certain way at a specific temperature.
These calculations are very detailed and take a long time to do by hand. And because there are many factors that all interact, it would take a long time to learn each material and the effect it has on a glaze. So potters have created a variety of computer programs that simplify the analysis and formulation of glazes.
You can learn more by taking the self paced on-line tutorial called Glaze Teach. http://www.matrix2000.co.nz/GlazeTeach/Index.htm The writers of this tutorial offer a glaze calculation software program called Matrix.
Or visit DigitalFire http://www.digitalfire.com, a website that explains glaze chemistry and sells a software program called Insight to help automate this glaze analysis process.
So, if this whole thing looks interesting to you, you will probably want to explore the world of glaze chemistry more. On the other hand, if it makes your eyes glaze over (no pun intended), you probably should stick to commercial glazes or trial and error. Happy glazing!
(note, the numbers in the above chemical formulas would properly be expressed as subscripts)
Browse our huge selection of glazes.