Layering Is a Conversation, Not a Gamble

Layering glazes is one of the fastest ways to make a home studio feel genuinely exciting. A soft rutile over a deep tenmoku, a cobalt blue brushed across a satin white, a speckled oatmeal under a copper green — the results can be extraordinary. They can also be a fused glass puddle welded to your kiln shelf.

The reason is simple chemistry. Every glaze is a recipe of silica, alumina and fluxes designed to melt at a particular temperature. When you layer two of them, you are not just stacking colours — you are adding their fluxes together. Boron, sodium, potassium, zinc and lithium all lower the melting point and increase fluidity. Two well-behaved glazes can combine into one that is far more mobile than either was alone. Understanding that is the difference between a happy accident and a chipped shelf.

Begin with Test Tiles, Always

Before a new combination touches a finished pot, it should touch a tile. Make yourself a batch of small test tiles — roughly 5 x 8 cm, pierced with a hole so you can hang them, and pressed with a little texture so you can see how the glaze breaks over edges and into recesses. Use the same clay body you throw or handbuild with, and bisque them to your usual temperature.

Then work through combinations systematically:

• Direction matters. Glaze A over B is rarely the same as B over A. Test both.
• Vary the ratio. Try roughly 1:3, 1:1 and 3:1 by thickness, not by guesswork.
• Keep the base consistent. If you always apply three coats of the bottom glaze, keep it that way so you are only changing one variable.
• Fire the tiles exactly as you fire your work — same kiln, same schedule, same load density if you can manage it.

Six to ten tiles per session will teach you more than a year of hopeful experiments on pots.

Thin Coats, Counted Properly

Most runny disasters are not the glaze's fault — they are a thickness problem. A glaze applied too heavily has more flux per square centimetre, and it will move.

If you are dipping, aim for a specific gravity around 1.40 to 1.50 for most stoneware glazes, and check it with a hydrometer rather than by eye. A quick dip of two to three seconds is usually plenty. If you are brushing, thin the glaze to the consistency of single cream and apply three thin coats, letting each one dry to a matte finish before the next. One thick coat is the single most common cause of crawling, shivering and runs.

A useful check: if the dried glaze layer looks chalky and opaque, it is probably right. If it has fine cracks across the surface, or flakes when you run a fingernail over it, it is too thick — wash it off and start again.

Know Which Glazes Want to Run

Some glazes are naturally stiff and forgiving, and some are not. Learn to predict which is which.

• Likely to run: high-gloss, high-boron glazes; anything with a lot of frit, zinc or lithium; saturated copper reds; most commercial clear glazes; glazes described as "fluid" or "reactive".
• Likely to stay put: matte glazes, anything high in alumina, and glazes containing tin, titanium or significant zirconium.

A good rule of thumb for layering is to put the stiffer glaze on the bottom and the runnier one on top. The stiff layer acts as a dam, catching the mobile glaze before it reaches the shelf. Putting a glossy, flux-rich glaze underneath a fluid one is asking for trouble.

Keep Records You Will Actually Use

A notebook beside the wheel is worth more than any glaze book on your shelf. For every test, record the date, batch number, clay body, bisque temperature, specific gravity, the order and number of coats, and the full firing schedule including soak time and cooling. Then write down what happened.

Number each test tile with a permanent marker or a china pencil on the unglazed reverse, and photograph it in daylight next to a ruler or a plain card. A phone camera and a shared folder is perfectly adequate. Six months later, when a customer asks for "that green one with the speckles", you will be glad you did.

Note the small things too: whether the kiln was tightly packed, whether you opened the lid early, whether the day was humid. All of it affects the surface.

Protect the Shelf and Learn From the Failures

Assume that something will run eventually, and prepare for it. Coat shelves with a good kiln wash, or use a sacrificial layer of alumina hydrate. Set pots on thin wasters or catch plates so that any drip is caught and can be snapped off afterwards. Keep work at least a centimetre clear of elements, and never place a heavily layered piece directly above another pot you care about.

When a glaze does run, resist the urge to blame the recipe. Reduce the total thickness first. Consider dropping your peak temperature by 10 to 15°C, or shortening the soak. Try reversing the layering order. Change one variable at a time, and fire another set of tiles. Over a few months you will build a personal library of combinations that behave beautifully — and a studio habit that keeps your shelves, and your pots, intact.