Switching to an L-shaped design allows you to learn more from your test tiles.

I have been testing glazes for many, many years and have either developed all of my own glazes or adjusted others to suit my glaze application methods and firing schedules. I prefer to use this L-shaped test tile design that I first came up with in graduate school when I taught the Raw Materials Lab class for my graduate assistantship under Val Cushing at Alfred University. During the class, we conducted tests using the conventional methods. After the fusion button tests, where we dropped mounds of ingredients into small divots in a flat slab tile, I thought, who would ever apply a glaze in this manner? Is this really applicable for ceramic artists? So, I conducted the tests again with the same ingredients in the same percentages, but utilized my L-shaped test tiles. The results were very different and much more meaningful. Instead of a series of glassy bubbles, they looked like glazes. I moved on to other material tests, adding lithium carbonate in 10% increments into 100% nepheline syenite with 2% copper carbonate added as a colorant to see the results better (see 5). I have used this method ever since for all of my glaze testing.

This L-shaped test tile design is ideal for glaze testing for several reasons. It effectively tests the influence of adding an ingredient to a glaze in five consecutive increments (0%, 10%, 20%, 30%, 40% in my examples) all on one test tile. This test tile clearly reveals how much and to what extent the ingredient influences the glaze. It also tests vertical flow and horizontal pooling, how the glaze breaks over texture, how it looks over a slip if that is desired, and very importantly, all the testing information is written on the back of the tile with an underglaze pencil, so it cannot be misplaced or lost.

The next test tile example (see 6) shows the influence of magnesium carbonate in 10% increments on a simple base glaze to develop a beading glaze. The base glaze is mixed up consisting of 80% nepheline syenite + 10% EPK kaolin + 10% silica = 100%. A brush stripe of the base glaze is made on the far left side. Then, 10% magnesium carbonate was added to that base glaze, mixed thoroughly, and a brush stripe of that mixture was made next to the previous one. Then, another 10% magnesium carbonate was added to that mixture, making it a total of a 20% addition of magnesium carbonate. It was thoroughly mixed and a brush stripe of that mixture was made. I repeated this process of adding 10% magnesium carbonate to the base glaze, mixing it, then brushing a new stripe onto the tile two more times to test for the 30% and 40% additions. After the test tile is fired in one of your normal firings, assess the results, choose which stripe you like best, and make up your new beading glaze.

Making the Test Tile

Wedge around 5 pounds of clay. Flatten each side as you rotate the block of clay and pound it down on a table to form a square-shaped block.

Gently toss it out on a canvas-covered table at a 45° angle to elongate and further flatten the block of clay to form a thick rectangular slab. Rotate and flip the slab of clay over with each toss. Continue to flatten the sides to keep it rectangular.

When the slab is about 2 inches thick, toss it out in one direction only to elongate it to just over 12 inches long. Elongate the short side until the slab is around ½ inch thick.

Set the slab roller to roll out and level the clay slab to ¼ inch thick or slightly thinner, or use strips of wood to gauge the thickness while rolling out the slab using a rolling pin.

Place the slab so that the 12-inch-long side is along the slab roller and roll it out to however long the clay will roll out.

Smooth out the canvas texture on the top side using a metal rib. This will make it easier to write on the back of the tile with an underglaze pencil. Set the slab on a ware board with the smoothed side down.

1 Cutting a right angle on the slab using a piece of paper as a guide.2 Making the measurement marks to cut out the 6-inch square tiles.

Cut a straight line down the 12-inch-long side as close to the edge as possible. This will be the bottom edge for your tiles. Rotate the slab and align a piece of paper with this cut and as close to one of the short sides as possible. This will give you a right angle to mark your next cut line. Set a straight edge along the paper’s edge and cut on this line (1).

Measure in from the first cut side and mark 2 sets of 6-,12-,18-, and 24-inch lengths, according to how long the slab extends (2).

3 Test tiles with added white slip and wavy incised lines made using a fork for texture. Cut the slab into six equally sized tiles.4. Tiles bent into an L-shape. After running a finger along the center crease to compress the angle and flatten the bottom, leave the tiles to dry.

Measure in from the second cut side and make two sets of marks at 6-inch and 12-inch lengths. Cut on these marks using the ruler to make the 6-inch square tiles.

If you want to see how the glaze responds over a porcelain or colored slip, as well as the clay, apply the slip in the middle of the slab, dividing the tiles into half slip and half plain clay.

Drag a fork across the slab in a wavy pattern to create four textured bands through the clay: two bands on each tile, one through the top or applied slip and one through the bottom or regular clay (3).

Separate the tiles and bend them in the middle to create the L-shaped tiles. Set them up against a 2×4-inch board for support. Run your finger down the center crease to sharpen the angle and flatten the bottom, which will help the tile stand upright on one side while firing (4). Bisque fire the tiles when dry. Test glazes with up to five incremental single ingredient changes. This test tile design is excellent to test and adjust a glaze to raise or lower its melting temperature, control the melt and flow, adjust the surface quality, develop a particular glaze effect, and more. It can dramatically reveal the influence of a particular ingredient added into a base glaze in increasing amounts. Using this L-shaped test tile design can help to develop many new glazes or tweak and improve old ones.

5 Test tile for base of nepheline syenite and copper carbonate with 0%, 10%, 20%, 30%, and 40% lithium carbonate (left to right).6 Beading glaze test tile showing incremental 10% additions of magnesium carbonate from left to right.

the author Steve Loucks is a studio potter and retired professor from Jacksonville State University, Alabama, where he taught for over 26 years. He has conducted many demonstration workshops and lectured on glaze development. Recently, Steve self-published a book, Glazes from a Potter’s Perspective: A Simple, Kitchen-Method Approach to Glaze Development. To learn more, visit

This article was excerpted from the October 2018 issue of Ceramics Monthly.