Glaze Tests and Test Tiles

There are many benefits to having test tiles for glazes. It allows you to test combinations of glazes easily and inexpensively, and serves as a permanent reminder of what a glaze looked like on a certain clay. Otherwise, if you’re like me, you forget very quickly.

Most people use some form of test tile for testing glazes. But some use small bowls or other forms. I often use “seconds”, so I can see it on an actual piece but not really care how it turns out. But this doesn’t store well. So, knowing that there are many benefits to having a standard set of test tiles, I set out to see what great ideas people had. Now I am really inspired to get organized with my testing, and I hope you will be too!

General ideas, regardless of the shape:

    • Texture: It is generally a good idea to add texture to part of your test piece so you can see how the glaze will respond to texture and carvings.

    • Holes: Put a hole somewhere to hang your tiles on a wall board or to hang off the handle of the bucket.

    • Marking: Put some type of word or code that reminds what glaze this was, what clay it was on, and perhaps even firing temperature. Or simply number them and keep the details in a notebook by number.

One way to number is with one of those adjustable rubber stamps (the ones with wheels to change the number).

  • Dipping: Usually best to do three dips. First dip covers the whole area. Second dip covers 2/3 of the area. Third dip covers half the previous dip. So you have 1/3 with 1 coat, 1/3 with 2 coats, and 1/3 with 3 coats. (Keep the thickest coat the furthest from the kiln shelf in case it runs)

Layer multiple glazes:

For example, take a circle, dip 1/3 in glaze 1, dip 1/3 in glaze 2 (overlapping glaze 1), dip 1/3 in glaze 3 (overlapping glaze 2 on one side and glaze 1 on the other). So you get 3 solid colors, plus 3 overlap colors. Another example: take a square, dip ½ in glaze 1, turn, dip ½ in glaze 2. At this point you have two solids, plus 1 solid over the other. For the 4th section you could try layering the opposite way (glaze 1 over glaze 2 instead of glaze 2 over glaze 1), have an underglaze coat, or introduce a texture or a third color.

Other layering effects:

For example, undercoat a section with underglaze, or paint an iron oxide and a cobalt oxide line so you can see if the oxide runs or stays put during firing.

What Shape to Use?

  • Rectangular tiles

  • Rectangular tiles with a bend (L shaped) so you can see the glaze on a vertical and a horizontal surface.

  • “T" shaped tiles made from an extruder. Make one side of the leg of the T smooth, the other side a texture. Do the three dips with the leg of the T. Lay it upside down (on the top part of the T) to fire, which will help to catch runny glaze, or if you glaze the whole inside L also give you an idea of what the glaze acts like on a flat surface.

  • Throw wide, shallow, bottomless vertical sided bowls. Leave a "flange" on the inside and outside bottom. Tool one side when dry, then cut vertically. This gives you a bunch of upside down "T" shapes with a slight curvature.

  • Throw a ring about 10" diameter but clay is only 2" from the perimeter and the inward leaning wall about 2" high too. Then when it is leather hard cut it into maybe 8 pieces like a pizza that way you'll have stand up tiles that will behave as the walls of your pots behave.

  • Throw a wide cylinder without a bottom about two or three inches tall and put two or three grooves in it. When it's leather hard, cut it like a pie, in four, eight etc pieces depending on how wide I want it to be and to make identical tiles. Each "slice" stands by itself and can be easily dipped in different ways at different angles to show different thickness of the glaze.

  • Extrude a hollow form with a hexagonal die, use a serrated rib to texture one side. When leather hard, cut into logs. You have a piece that stands and has plain and textured test's of the glaze

  • Small cups (pinched or thrown) with glaze on the interior

  • Using cookie cutters make round tiles with a decent sized hole a little in from one edge. Leave one half smooth & flat and use any method you like to create a raised pattern of some sort on the other half, so you get an idea of what the glaze would look like on an incised or impressed surface.

  • Extrude a hollow square or hexagonal form to create a long tube. This gives 4 surfaces to do different things to. One side is scratched with a fork or a serrated rib for texture. One side receives black underglaze, one red underglaze, etc. and the last side left plain. (You can vary this, add oxide lines to see how they react) Slice into sections about 3 inches long and punch holes.

Hanging and storage:

  • Hammer rows of nails into a large board and hang the tiles on them. If you use long nails, each one will take several tiles. You can sort them by color, texture, etc.

  • String several together (for example, all the tests that you like on a certain clay, or all the tests of a certain color range), then hang the groups on nails.

  • Get a large box to keep those you will never want to use again! Never throw glaze tests out... You THINK you will always remember the duds, but it is surprising how fast you forget the results even if you remember testing a certain glaze. Heck, sometimes I can’t remember what glaze I put on a piece as soon as I take it out of the kiln! Of course I have drawn the pot and written the glaze, but my drawing is so poor I can’t match it to the right pot! (Yes, I am extremely drawing challenged.)

Mixing test glazes:

  • Most people mix 100g batches of test glazes, although some that use bowls or larger pieces go up to 300g. Often people make a larger batch, then break it down to smaller pieces for the addition of different colorants (stains and oxides).

  • With test batches, as with full batches, it is useful to let the glaze sit for 2 days to fully absorb, then re-mix.

  • Many people use old blenders or stick blenders for mixing test batches. They mark off with a magic marker where enough water for a 100g test comes, so they can just put in the water and add ingredients as they are weighed out. (Tip: Use a piece of masking tape until you are sure enough to draw your line with a magic marker.) Another reason to mark where a 100g or a 200g batch of wet glaze comes, is so you can can add that much base glaze before adding colorants or other ingredients.

  • Some think it’s important to screen after each coloring oxide addition for smoother results. Inexpensive test sieves are available that fit over a jar or half pound margarine cup. Also, some people use permanent coffee maker filters as screens.

  • If your recipe calls for .5g and your scale is only accurate to 1g, weight out 1 gram onto a piece of paper. With a blade, smooth it into a flattened square, and separate it in half by eye. Or into quarters.

My winner?

Since I have an extruder, I’m going to extrude multiple sided hollow shapes so I can cram as much information as possible on one tile. I like the idea of being able to have a textured side (or maybe even two textures going different directions), an underglazed side, and a side with oxide lines. I also like that I can use the same shape for layering glazes; instead of dipping in the same glaze 3 times, dip in different glazes.

Hopefully you're found a method here which catches your imagination and works with your work style too.

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Sparks, NV 89434
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