Glaze Layering for Beginners: How to Test Combinations Without Ruining a Kiln Load

Layering glazes is where pottery stops feeling like following a recipe and starts feeling like cooking. You dip a mug in a satin white, let it dry, dip the rim in an iron tenmoku, and the kiln hands back something that looks like a storm front. It is also how a beginner ends up with two pots welded to a kiln shelf and a fresh dislike of the whole idea.

Test tiles take most of the risk out of it. Not because they are tidy, but because they let you be wrong at a scale you can afford. What follows is a working method: how to make tiles worth firing, what to write down, and which combinations to keep away from your good work.

Make tiles that tell you something

Roll a slab to about 6 mm (a quarter inch) and cut rectangles roughly 5 by 8 cm. Punch a hole near one end with a straw so you can hang the finished tile on a wire later. While the clay is still leather hard, scratch a number into the back with a needle tool. Pencil and marker burn away in the bisque; a scratched number does not.

Use the same clay body as your pots. A glaze that sings on porcelain can look muddy on speckled stoneware, and a tile made from a scrap bag tells you almost nothing about the clay you actually throw. If you work in two bodies, make two sets of tiles. Label them with a clay code as well as a number, because “tile 4” means nothing when you have a drawer full of tile 4s.

Make more than you need. Twenty tiles is an afternoon and a fistful of clay. Stand some of them upright on their edges instead of laying them flat. A flat tile shows color and surface; a leaning tile shows movement, and movement is what welds pots to shelves.

If you want to test how a glaze behaves on a rim or a foot, make a few small pinch cups or thrown cylinders as well. Tiles are flat, and pots are not. A combination that looks calm on a tile can still pool in a mug’s interior or break over a rim. The tile is the first filter, not the final answer.

Write it down before the kiln erases the evidence

By the time you unload, you will not remember whether tile 7 was two dips of oatmeal with one dip of blue over the top, or the other way around. Record every tile before it goes in. A small notebook is enough; the format matters less than the consistency.

• Clay body and bisque cone
• Glaze names, manufacturers, and product codes, written out in full
• Application: number of dips or brush coats, and how many seconds each dip lasted
• Which glaze went on first, and how long you let it dry in between
• Where on the tile the layers overlap
• Firing cone, atmosphere, and which shelf the tile sat on
• Date, plus a one-line sketch of the tile layout

Photograph the finished tile next to its notebook page. Ceramic surfaces shift under different lighting, and a phone photo taken in the same spot every time will save you from arguing with your own memory six months later.

A simple naming system helps when a combination works. Try something like 240615-SW-OAT-BLU-2.5: date, clay code, first glaze, second glaze, and dip time. It looks ridiculous until the day you need to remake a winner and cannot remember whether “the blue one” was cobalt or rutile.

The rules that keep a load off the kiln repair pile

Commercial glazes are formulated to be applied at a specific thickness. Layer two of them and you are applying roughly twice that thickness, often more in the overlap. That extra glass is what runs.

• Count total thickness, not layers. Two three-second dips are already generous for most glazes. When layering, shorten both dips rather than stacking full ones. If a glaze normally gets a four-second dip, try two seconds for the base and one or two seconds for the top.
• Do not layer two runny glazes. If a glaze alone drips off a vertical tile, it will drip harder with another glaze on top of it. Save runny glazes for the inside of a bowl or the top of a tile you are willing to sacrifice.
• Let each layer dry fully. Dipping a wet surface drags the layer beneath and leaves bald patches and crawling. A hair dryer on low can speed this up, but let the glaze cool before the next dip.
• Keep the second layer off the foot. Glass collects at the lowest point of a pot, which is exactly where the shelf is. Wax the foot ring before you start, and wipe the bottom clean even if you think you missed it.
• Mind the firing range. Two glazes with different cone ranges can underfire or overfire together. A cone 6 glaze over a cone 10 glaze will not magically become cone 6. Check both labels and test at the cone you actually fire to, not the one you wish you fired to.
• Be careful with high-iron, high-cobalt, and high-zinc glazes as top layers. They can behave unpredictably, alter the color below, create metallic scum, or flux the glaze beneath and cause unexpected runs. A little goes a long way, especially at the rim.

If a glaze runs on its own, layering it does not make it polite. It makes it a bigger problem.

Read the tile before you fall in love

Once the kiln is cool, do not just pick the prettiest tile and start glazing mugs. Read each one like a mechanic reading a spark plug. The surface tells you what happened, and the edges tell you what will happen on a vertical pot.

• Color and break: Does the top glaze change the base glaze’s color, or just sit on top? Iron tenmoku over white often pulls the white toward cream; blue rutile over oatmeal can shift toward green or gray with speckles.
• Surface: Is the overlap glossy, satin, or dry? A glossy top over a matte base can look like cracked ice, which is beautiful on a tile and awful on a plate if it feels rough.
• Movement: Look at the bottom edge of a leaning tile. If the glaze has crept past the clay, shorten your dips or fire on a waster slab.
• Fit: Crazing may show up immediately or days later. Write the date you checked it, and check again after a week. A glaze that crazes on a tile will craze on a pot.
• Flaws: Pinholing, crawling, blistering, and shivering are all information. They usually mean the layer is too thick, the glaze is too high in one material, or the firing schedule cooled too fast.

Keep the ugly tiles. They are the ones that teach you where the edge is. A tile with a run you can trace back to a four-second dip is more useful than ten perfect tiles with no notes.

A starter test plan for your next bisque load

You do not need to test every glaze in the studio. Pick three base glazes you already trust and three accent glazes you are curious about. Make a set of tiles that covers each glaze alone, each base with each accent on top, and each accent with each base on top. That is twenty-four tiles, which sounds like a lot until you realize it is one afternoon of rolling and dipping.

Use short dips for every tile. Stand half of them upright. Fire them together, on the same shelf if possible. When they come out, arrange them in a grid on a table and photograph the whole grid. You will see patterns faster than reading twenty-four notes one by one.

For example, a simple test might give you oatmeal as a base and iron tenmoku as an accent. Oatmeal alone is a soft, satin tan. Tenmoku alone is dark brown where thick and amber where thin. Oatmeal under tenmoku can give you a warm, breaking rim that looks like toasted bread. Tenmoku under oatmeal can turn muddy and stiff. That one test tells you the order matters.

Another useful pair is satin white and blue rutile. Blue rutile over satin white often gives a soft, watercolor blue with speckles. Satin white over blue rutile can

Photo: kropekk_pl / Pixabay