How to mix pottery glazes

Česky: Barevný vzorkovník glazur. Ateliér Hruš...

Česky: Barevný vzorkovník glazur. Ateliér Hruška, Praha Smíchov. (Photo credit: Wikipedia)

How do You Read a Ceramic Glaze Recipe?


Ceramic glaze recipes are listed in two standard forms.
The first is the Percentage Recipe Form. Here is an example: Tenmoku
Custer Feldspar 50.5% Whiting 12.1% Silica 20.6% EPK Kaolin 5.6% Barium Carbonate 2.3% Zinc Oxide 2.3% Red Iron Oxide 6.6% Total: 100%
In the percentage form, the percentages add to 100%. You use the percentages to figure out the amount of each material to add. So, for this example, if you wanted to mix 1000 grams of Tenmoku glaze, you would add .54 * 1000 = 540 grams of Custer Feldspar, 0.13 * 1000 = 130 grams of Whiting and so on to get a total of 1000 grams.
The second form is the Base Glaze Recipe Form. Here is an example:
Custer Feldspar 54% Whiting 13% Silica 22% EPK Kaolin 6% Barium Carbonate 2.5% ZincOxide 2.5% Total: 100%
Add : Red Iron Oxide 7%
In the Base Glaze Recipe Form, the top, base section, has percentages that add to 100%. But, there is an additional Add section that will contain one or more colorants to add to the 100% base section.
This is a little different, because if you add all of them together you get 107%. This form was established because potters sometimes start with a clear base glaze and add different colorants to it to make other ceramic glaze recipes.
So, if you wanted to make 1000 grams of glaze you have to do some math. Because your recipe total is 107 you need to multiply each percentage by 1000 / 107 = 9.35. This scalar of 9.35 will convert the recipe to the 1000 gram amount.
The new scaled recipes is:
Custer Feldspar 505 Whiting 121 Silica 206 EPK Kaolin 56 Barium Carbonate 23 Zinc Oxide 23 Red Iron Oxide 66 Total: 1000

How Do You Measure the Ingredients?


You need an accurate scale to measure out ceramic glaze ingredients. You can use a traditional triple beam scale or a digital scale. Either way, the scale you use must be accurate to 0.1 grams.
To start, you need to level the scale to 0 grams.
If the scale bed is flat, you need to put a small container on the scale to hold the ingredients you want to measure. But, you don’t want the weight of the container to count in the weight of the glaze material. To discount for the weight of the container you must level the scale to 0 grams.
To do this, place the empty container on the scale. If you are using a triple beam scale, the zero knob should be close to the scale bed. Move all measure beams to 0 and then turn the zero knob until the scale is balanced. If you are using a digital scale it will usually balance to 0 if you turn the scale off, put the container on the scale bed, and turn the scale back on. Just make sure the scale says 0 grams.
Once you have your scale balanced to zero and you have all your ingredients, you are ready to mix!
Go through the glaze recipe items one by one, measuring the correct amount of each ingredient and putting the dry material in a bucket. After you are finished with each material, make sure you clean out the container on the scale and clean whatever scoop you are using for the materials. (Always avoid material contamination. The residue of a colorant on a spoon can contaminate another material and drastically change the result of your next glaze).

Mixing the Dry Glaze with Water


Now that your ceramic glaze recipe is mixed, your next step is to mix the dry material with water. (Glazes are mixed with water to make them easier to apply to pots. After pots are bisque fired, the chemical water is driven out of the clay. This makes the pot suck in water like a sponge. When you put a wet glaze on a bisque fired pot, the pot sucks in water from the glaze and with it some of the glaze itself. This makes the glaze stick to the pot. It would be very difficult to get a dry powdered glaze to stick to a pot.)
Normal glazes are usually 50% water. This means that a normal 1000 gram glaze will take 1000 Milliliters of water. This is normal, but your glaze may require more or less water. So, I usually start with a little less.
There are about 237 milliliters in a cup of water. So, for 1000 grams of dry material, I usually start with about 4 cups of water.
Put the water in a large bucket and slowly pour in the dry material while stirring the water.
Stir the glaze until it is an even consistency with no chunks of material. To remove chunks you can push the glaze through a sieve (I usually push the glaze through an 80 mesh screen twice). Your final glaze should be about the same thickness as cream. But the real test is to put the glaze on a pot. Dip a pot in the glaze and remove it. Let it dry and chip some of the glaze off of the pot. The glaze should be no thicker than 1/8th of an inch. If the glaze is too thick, add some water. If the glaze is too thin, let it settle over night and then pour some water off of the top.
Make sure that you store your glaze in an air tight container. The water can quickly evaporate making the glaze too thick.
Stir your glaze every week. If the glaze is left too long without stirring, the material can settle in the bottom of the bucket and become very difficult use.
It is also a good idea to label your glaze buckets (It is amazing how fast I can forget what glaze went in what bucket!)

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