Mixing and Testing Pottery Glazes: A Great Way to Expand Your Understanding of Glaze Chemistry

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.

Glaze Making

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.

Glaze Testing

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.

Comments
  • I like testing one formula on different clays and at different temps.

  • Nice and clear process, but when you talk about”appropriate consistency” I would like to know a value(s) of density (baume degree) in order to see what milky aspect means.
    Thank you for cooperation.
    Christian

  • Like Connie said it is important to test your glazes under all circumstances that you use in your studio work. Glazes change with the slightest variation in measurements and from firing to firing. Doing multiple tests and taking detailed notes with help you in your discovery. One way to play with your glazes is to put 1 cup of your glaze base in 10 pint containers for 10 tests. Then take a mason stain or other colorant that you choose and add a teaspoon of colorant to the all the cups and start increasing the amount incrementally into each. So, the first cup will have one cup of glaze and 1t.of color and all of the other cups will have one cup of glaze and the number of teaspoons of colorant will be determined by what cup you are on. Cup2/2t.;Cup3/3t.;and so on when all are filled mix very well and sieve if desired. Now when you make your test tiles you should make them at least 3 inches tall with an incised design or a line or two to show you when the glaze will break on the High points or edges. To get a good ideas of what a thick coat will do on a pot try it out on a test tile. I have two easy ways to make a bunch of test tiles fast…(1)put 2-3lbs of clay on a bat on the wheel and center. Once centered open down to the bat leave clay at the bottom of your walls to have them something to stand on. Open the cylinder wide and refine the rim and if you want to do your decoration on the wheel now’s your time. Wire off but leave it on the bat whole until it is nearly leather hard, then cut the wide low cylinder into 2 inch wide pieces. You should put your initials, glaze batch and test number and if you’re smart the date too, on the back of the tiles. Remember not all of your tiles are getting the same color saturation test so keep ten of your tiles for that test and

  • Oops I wasn’t done and hit the button. so here is what comes after my hanging in the wind “and” So instead of trying to remember exactly what I had planned to say when I was so rudely interupted by a rogue submit button, I will start my thought again.

    So once you have your test tiles incised and labeled fire them to bisque.
    The other way I have made test tiles it to roll out a large slab of clay to about 1/2 inch and use a ruler or straight edge and fettling knife to cut 2″ wide strips of clay then turn the slab so you can cut across the 2″ strips to make 6″ cuts so you will end up with a 2X6″ strip. Now take each strip one at a time and fold 2 of the 6″ up and turn the strip over and pull up the unfolded edge and fold that backward to the middle of the first fold now you should have am upside-down T lean the upright leg of the T towards the folded clay behind it and press at the fold to set the position. This is much easier to do than to write. Now incise lines or a design into you tiles and mark with your vital statistics on the back and fire to Bisque.

    Now for the fun part. Once you have all your test tiles ready for your experiment remix your glazes and if they are thick like yogurt add water slowly until they are more like heavy cream.
    Take your first tile and dunk it one time deep into the 1st glaze remove quickly. That should have covered most of the tile. Now dunk the same tile into the same glaze 2 more times not going in as deep as the one before so the thicknesses will be noticable. So you will end up with 30 different results on the 10 test tiles. This is a fun way to play with color and you can use your imagination to lead to the way.
    Artfully Yours!
    Jenni

  • I check the specific gravity of my glaze once the firing proves it to be what I want and mark my container. The next time I mix that glaze I will be able to repeat the consistency so my new glaze will be like my previous one.

  • Richard Zakin is one of my favorite ceramic authors. There is, however, one item I do differently; after all the dry ingredients are measured and together, in a separate clean container is water where the dry ingredients are gradually added and mixed. I use a plastic toilet bowl brush from the dollar store to mix and leave it in the bucket as it is 100% plastic. It doesn’t rust or disintegrate and sieves as it mixes. Water is then added to the consistency preferred for that glaze. Adding dry to the wet mixes much easier than wet to dry. Thanks for all the great aritcles C.A.D.

  • It is also a good idea to let your glaze sit for a day to let all of the particles fully hydrate before dipping your test tiles.

  • Placing test tiles with the same glazes and clays in different places in the kiln during the same firing could give an idea of any heat variations, i.e. cone reached and/or heating and cooling rates. Some glazes can be very sensitive to these variables.

    They can differ from the top, middle, bottom of the kiln as well as the middle of the shelf and the shelf edge. These differences may not be more than a cone or less than a few degrees per hour, but some glazes will react noticeably.

    It’s good to experiment with placement to see the variations, if any. It’s very helpful if you want consistent results for matching pieces.

  • For help with converting glaze recipe percentages into the correct gram weight, you can use GlazeCal. Even for those of you who know the formula, GlazeCal makes the process much easier, saves time and reduces error. I use it in my studio every time I make a new glaze. The math is easy, but I’m glad I don’t have to do it and can spend my time on the things I love – like making more pots. Hope this helps!

    http://www.GlazeCal.com

    Christeena

  • I’m trying to find a form (or webpage) giving different ranges to be used for all chemicals used in gas fire cone 10 kiln. (ie: G200 25%-45% for cone 10/ EPK 0-20% for cone 10). Thank you

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