Brew, Burn, Sieve: Six UK Potters Making Glazes from Kitchen Waste

In a shared studio in Bristol, a jar of spent espresso grounds sits next to a bag of feldspar. It is not destined for the compost. It is going to be dried, burned, sieved and turned into a glaze test. Across the UK, a quiet strand of studio practice is doing the same thing with wood ash, eggshells and local stone: taking what would otherwise be waste and asking whether it can make a useful surface.

This is not about being smug or saving the planet one mug at a time. It is about curiosity, local colour and the satisfaction of building a glaze you can actually repeat. It is also about being honest: kitchen waste is variable, dusty and sometimes chemically unpredictable. The makers doing it well are not chucking ash into a bucket and hoping. They are testing, labelling and keeping notes like lab technicians in aprons.

Why kitchen waste works in a glaze

Wood ash, coffee ash and calcined eggshells bring calcium, potassium and other fluxes to a glaze. Local stone and clay waste can add silica, alumina and colour. In the right proportion, these materials help a glaze melt, mature and fit the clay. In the wrong proportion, they can craze, run, blister or turn into a dull sheet of disappointment.

That is why every maker below starts with small test tiles and a known base recipe. They change one thing at a time. They fire the same way. They write it down. If you fancy having a go, copy the habit before you copy the recipe.

The six makers

1. Nell, Bristol — spent coffee grounds

Nell collects used grounds from a café near her studio, dries them thoroughly and burns them down to a fine ash in a small outdoor burner. The ash is sieved to 80 mesh and tested in a simple glaze: 30% coffee ash, 30% feldspar, 20% whiting and 20% china clay. Fired around 1240–1260°C, it gives a speckled, warm brown surface that can run if applied thickly. Nell uses it on the outside of pots and keeps the inside lined with a trusted liner glaze. 'It smells terrible when it burns,' she says. 'Do it outside. Seriously.'

2. Iwan, Powys — wood ash

Iwan burns only clean, untreated wood in a wood burner. He sieves the ash, stores it dry and tests it in a glaze of 40% wood ash, 30% feldspar, 20% ball clay and 10% whiting at 1260°C. The results shift with the wood — oak ash behaves differently from ash from softwood — so he keeps separate jars labelled by date and source. No coal ash, no painted wood, no mystery bonfire debris. 'If you would not burn it in your living room, do not put it in a glaze,' he says.

3. Morag, East Lothian — eggshells

Morag collects eggshells from a local café, washes them, dries them and calcines them outdoors. Calcined eggshell is mainly calcium oxide, so it needs respect: gloves, a dust mask and no leaning over the tray. She tests it as a partial substitute for whiting in a base glaze, for example 20% calcined eggshell, 30% feldspar, 30% quartz and 20% kaolin. The result is a soft, creamy matte that she describes as 'nice on a bowl, terrible on a mug if it crawls'. It is a test glaze, not a default liner.

4. Ceri, St Austell — china clay waste

Ceri works near the Cornish clay country and uses small quantities of fine china clay waste from a local pit, with permission. Her test recipe is 25% clay waste, 35% feldspar, 25% quartz and 15% whiting, fired to 1260°C. The waste brings body and a slightly smoky colour. She warns that industrial waste is not automatically safe: know what it is, where it came from and whether it contains anything you would not want in a studio. 'Local' is not a synonym for 'harmless'.

5. Dev, Norfolk — flint and chalk

Dev calcines small flint pebbles from the coast and combines the sieved material with local chalk in a test glaze: 20% calcined flint, 20% chalk, 35% feldspar and 25% ball clay. The flint adds silica; the chalk adds calcium. He fires test tiles at 1250°C and keeps a photo log. Calcining flint can crack stones and create dust, so he does it outside and wears proper respiratory protection. The best result so far is a pale, slightly opaque glaze with a pleasant dry surface.

6. Ailsa, Kent — seaweed ash

Ailsa collects clean, washed-up seaweed from a beach she knows, rinses it, dries it and burns it in small batches. She tests 20% seaweed ash with 30% feldspar, 25% quartz and 25% whiting at 1240°C. The ash brings a greenish tint and plenty of variability. Salt is the obvious concern, so she rinses thoroughly and keeps the tests small. 'It is a fun surface on a sculptural piece,' she says. 'It is not going anywhere near a dinner plate until it has been properly tested.'

Practical starting points for hobbyists

Start with a glaze you already trust and substitute only a small percentage of one material — 5% to 10% waste is plenty for a first test. Make at least six tiles from the same clay body. Label them with the recipe, application thickness and firing temperature. Fire them in the same kiln cycle as your control tile. Keep a notebook that records the source of the waste, the date, how you sieved it and what happened.

Do not assume a beautiful test tile is food-safe. Ash glazes can be rough, leachable or prone to crazing. Use them on decorative work or over a reliable liner glaze until you have proper evidence. If you want to make food-contact ware, stick to proven studio glazes or get expert testing advice.

Safety is the boring bit that keeps it fun

Wear a dust mask when handling ash, calcined eggshells, flint or clay waste. Keep dust down with water where possible. Do not eat or drink in the glaze area. Store waste materials in labelled, sealed jars. Burn only clean wood, paper or plant material — never treated timber, coal, painted scraps or plastics. Calcining eggshells and flint should happen outdoors, with gloves and eye protection.

And keep the test-tile mindset. The makers above are not wizards. They are just consistent. A jar of coffee grounds is not a glaze until you have weighed it, fired it, written it down and fired it again.