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Mixing and Testing Pottery Glazes: A Great Way to Expand Your Understanding of Glaze Chemistry
Posted By Richard Zakin On June 8, 2011 @ 7:34 am In Daily,Features,Glaze Chemistry | 10 Comments
Learning how different materials contribute to glazes is very important in expanding your abilities as a ceramic artist. And the best way to learn about glaze chemistry is to test, test, test. By testing lots of recipes and varying the ingredients, you can become familiar how ceramic raw materials behave and interact.
In today’s post, Richard Zakin explains his straightforward system for making and testing glazes. With this primer, you’ll be able to start testing away! – Jennifer Harnetty, editor.
For most ceramists the first experience of the technical side of ceramics takes place during glaze making. The process of glaze making is easily mastered if you have the right tools, follow an ordered procedure, and take the work seriously.
You Will Need
-An accurate scale calibrated in grams.
-A clean pan or bucket in which to weigh the glaze materials (your scale may come with such a pan). This is called the measuring container. Stainless steel salad bowls come in various sizes and make excellent measuring containers.
-A clean bucket in which to mix the weighed glaze materials. This mixing container must be large enough to hold the entire recipe.
-A good dust mask (with government safety approval).
-Water (for suspending the glaze).
-A fine sieve, either 50 or 80 mesh (50 or 80 strands to the inch).
-If you are making up a large amount of glaze (more than 2,000 grams) you will also need a coarse sieve (the type you can buy in the supermarket).
-A clean bristle brush (for pushing glaze through the sieve).
-A waterproof marker (for labeling the glaze container).
The Glaze Making Process
-Put on the dust mask.
-Locate each material in the recipe and make sure you have enough of it.
-Clean the scale and make sure it’s properly balanced before you begin work.
-Place the measuring container for weighing your materials on the scale. With no materials in the container, the indicated weight should be set at zero point. If not, adjust the tare compensation of the scale so that it reads zero.
-Weigh your first material.
-Place it in the container that you will use to mix and store the glaze.
-Weigh out each successive material and place it in the mixing container.
-Add enough water to make a mixture the thickness of cream.
-If you have a propeller mixer, use it at this point. Otherwise, mix the glaze with a stirring stick or a wire whisk. Once the glaze is properly mixed with water, you may remove your mask.
-Place a sieve supported by two sticks on top of another mixing container.
-Pass the glaze mixture through the sieve (you can use a stiff brush to force the glaze through the sieve). This homogenizes the mixture and gets rid of any lumps. If you have made up a large amount of glaze (more than 2,000 grams) it greatly speeds up the process to pass the mixture through a coarse sieve before using the fine sieve.
-Move the sieve over to the original mixing container and pour the glaze through the sieve once more. Double sieving insures a smooth mixture.
-Make a waterproof label for the glaze and place it on the container.
Unlike paint, glazes must be fired. Furthermore, glazes are transformed by the fire and do not have the same surface or color before they are fired as after.The kiln firing changes the characteristics of the glaze in a most profound way. The best way to track these transformations is to fire glazes first on a test tile. This will allow you to see what a glaze’s surface, color, and texture are after firing. The test tile should be fairly large and should have a character that is similar to your normal work. It is especially important to use the same clay and firing as you normally use in your work. Both of these strongly influence the character of the glaze.
The Testing Process
-Prepare a test tile.
-Thin the glaze with water to the appropriate consistency. For single color application this is liable to be the thickness of heavy cream. If you plan to use the glaze in a multiple-layer glazing strategy, the glaze (or glazes) should be thin and milky.
-Apply the glaze to the tile by dipping, pouring, or spraying.
-Fire the tile in a way consistent with your normal firing methods.
-Label the completed glaze test. Include its name, recipe (including colorants), firing cone, and the date. In a classroom or group situation include your initials for identification.
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