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A skin of glass

Why Doesn't a Glazed Pot Leak?

Fire clay hard as stone and it will still let water seep through. What makes a mug hold tea is the thin layer of glass melted onto the outside of it.

A glazed jug holdswater for years Because it is wearinga skin of glass
The pot is the strength; the glaze is the seal.

  1. Step 1 of 5

    A fired pot is full of holes.

    Water sittinginside anunglazed pot Seeps out throughthe gaps betweenthe grains

    Firing fuses clay grains at their edges but leaves gaps all through, and water works its way along them. That is exactly why an unglazed plant pot goes dark on the outside when you water it, and why it dries a plant out so quickly.

  2. Step 2 of 5

    Glaze is powdered glass, painted on as mud.

    1 Ground mineralsstirred intowater2 The pot isdipped and drieschalky and dull3 Only the kilnturns that coatinto glass

    The potter grinds up the ingredients of glass, stirs them into water, and dips the pot. It comes out coated in something dull and chalky that looks nothing like glass, and stays that way until the kiln.

  3. Step 3 of 5

    It has to melt at the pot's temperature, not at glass's.

    Plain sandMelts near1700°C — the potwould slumpfirstSand with a flux addedMelts near1100°C, whichthe kiln canreach

    Sand alone melts near 1700°C, far past the point where the pot itself would slump into a puddle. So the recipe adds fluxes — soda, potash, wood ash, once lead — which drop the melting point to something a kiln can reach without destroying what it is coating.

  4. Step 4 of 5

    And it has to shrink by exactly as much as the pot.

    The glazeMust shrink thesame amount aswhat is underitThe pot bodyOr the skineither crazesor flakes away

    Both cool from red heat and both contract. If the glaze contracts more it splits into the fine web of cracks called crazing; if it contracts less it is squeezed until it flakes off in slivers. The famous crackle finish on some pottery is that fault, deliberately dialled in.

  5. Step 5 of 5

    The colour comes from metals, and from the kiln's air.

    Cobalt: blue, whateveryou doCopper: green in air,red without it
    The kiln's atmosphere is part of the recipe.

    A few percent of a metal oxide colours the whole glaze: cobalt for blue, iron for the browns and greens, copper for green. Starve the same copper glaze of air while it fires and it comes out deep red instead — same recipe, different air.

    This is why old lead glazes mattered so much and why they are gone from tableware: lead melts beautifully low, and it also leaches out into acidic food.

The short version

A glazed pot holds water because a layer of glass is melted over its porous surface — and that glass must melt low enough and shrink at the same rate as the pot.

Try it yourself

Run a wet finger over the unglazed foot ring on the bottom of a mug, then over the glazed side. One drinks the water and one does not.

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