Why your glaze never looks like the bucket

Glaze day is the cruellest day in pottery. You have already thrown the pot, trimmed it, dried it, bisqued it — a week of work, minimum — and now you dip it into a bucket of grey sludge on the strength of a hand-written label and a memory of what that glaze did last time. Then you close the kiln and wait a day and a half to find out.

There is no undo. Which is why it is worth understanding exactly why the bucket tells you so little.

Raw glaze is not glaze yet

What is in the bucket is a suspension: finely ground rock and minerals — silica, feldspar, whiting, clay, and a few colouring oxides — held in water. It is closer to a milkshake of powdered stone than to anything glassy. The colour you see is the colour of that powder, wet.

What comes out of the kiln is glass. Somewhere above 1000 °C the fluxes start doing their job, the silica dissolves into the melt, and the whole coating turns liquid and then freezes into a transparent or opaque glassy layer bonded to the clay. Nothing about the appearance of powdered rock predicts the appearance of glass made from it.

What actually changes in the kiln

• Gloss. Raw glaze is always matte and chalky. Fired, it can be anything from a dry satin to a mirror, and that difference alone changes the perceived colour more than any pigment does.
• Colour development. Colouring oxides are shy until they melt. Copper carbonate is a dull grey-green powder that fires to a bright turquoise in an oxidised firing and a blood red in reduction. Cobalt is a nearly black powder that becomes the most saturated blue in ceramics. Iron can be honey, celadon green, or tenmoku black depending entirely on how much of it is in there and what the kiln atmosphere is doing.
• Opacity. Many glazes are opaque raw and transparent fired, so the clay body underneath suddenly starts to matter. The same glaze over a white porcelain and a dark iron-bearing stoneware can look like two unrelated glazes.
• Speckle. Manganese and iron speckle in the clay bloom through the glaze during the firing. You cannot see a single speck of it on the raw pot.
• Movement. A fluid glaze runs while molten. It thins on the rim and every sharp edge — which is where the colour “breaks” to show the body — and pools thick in the throwing rings and at the foot, going darker and deeper there.
• Layering. Two glazes over each other do not blend like paint. They react. The interface is often the most interesting thing on the pot and almost never the average of the two colours.

Test tiles help, and then they lie

Everyone tells you to make test tiles and everyone is right. A tile fired in your kiln with your clay is far better evidence than a photograph on a supplier’s website. But a tile is a flat rectangle, and your pot is not.

• A tile has no belly, so it cannot show you how the glaze pools where the wall turns.
• A tile is dipped once at one thickness; on a real pot the thickness changes with every shape change and every second you leave it in the bucket.
• A tile is usually fired standing upright and flat, so it never shows the way a fluid glaze thins over a rim.

This is why glazes that looked lovely on the tile can still surprise you on the pot: the tile told the truth about chemistry and said nothing about your form.

What a fired preview is for

Seeing a glaze rendered as it comes out of the kiln, on the actual curve you designed, answers a different question than the test tile does. Not “what colour is this glaze” but “what will this glaze do to this pot.” Where does the band sit relative to the belly? Does the pattern read at all once it wraps? Is the dark liner going to fight the exterior? Those are composition decisions, and you can make them in a minute instead of a firing.

The practical habit

Decide the composition before you touch the bucket. Know which glaze goes where, how far up the band sits, and what the interior is doing. Keep firing test tiles for chemistry. Photograph the fired result next to the design so that next time your memory has something better than a hand-written label to work from.

You can try the raw and fired toggle on the piece spinning on our homepage without an account, or read how the layer stack works.