Glaze Safety Checklist: 10 Things to Verify Before Firing Your Glazed Pieces

Glazing is the moment a pot stops being clay and starts behaving like chemistry. A glaze that looks flawless on a test tile can craze after three dishwasher cycles. A shelf that wasn't re-washed can weld a bowl to the kiln for good. None of it is dramatic, and none of it is difficult to avoid — it comes down to a handful of checks done in the right order, before the kiln lid goes down.

What follows is the routine used in working studios: verify the materials, verify the application, verify the load. Ten minutes at the bench saves a great deal more than ten minutes of ruined pots.

Start with the label, not the kiln

Every glaze ingredient has two identities: what it does in the bucket and what it does on the pot. Barium carbonate, cobalt oxide, manganese dioxide, copper carbonate, nickel oxide and lithium carbonate all appear in ordinary studio recipes, and most are hazardous as powders — which is exactly the form you handle them in. Fired into a well-melted glaze, many metals become locked into the glassy surface, but some, lead and cadmium among them, remain a concern on food-contact surfaces no matter how carefully the firing goes.

So before you open a new tub, read the supplier's label and the safety data sheet that comes with it. Note the hazards and keep that information with the glaze. A simple notebook is enough: supplier, batch, date, and where the recipe came from. If a container turns up without paperwork, treat it as suspect and keep it away from anything that will hold food.

Commercial glazes labelled for food use are a reasonable starting point, but the label describes the product, not your pot. Thickness, clay body, firing temperature and how well the glaze melts all affect the finished surface. Treat the label as permission to test, not proof of safety.

Separate food ware from decorative ware before you glaze

Decide which pots are going near food while the buckets are still closed. There's nothing wrong with a richly coloured, high-metal glaze — on the outside of a vase, or on a sculptural piece that never meets a spoon. It simply doesn't belong on a mug rim or the inside of a serving bowl.

Food-contact surfaces want a liner glaze you have fired and tested before, and a surface that is smooth, well melted and free of crazing. That fine web of cracks traps liquid and gives whatever is in the glaze a route into your tea. Cutlery marks, dull patches and a surface that catches under a fingernail are all good reasons to move a piece off the kitchen shelf and onto the display one.

A rough check at home: pour boiling water into the pot, leave it to stand, then look for cracks once it has cooled. Or wipe a wedge of lemon across the glaze, leave it a while and see whether the surface etches or dulls. These are indicators, not laboratory results. If you sell food-contact ware, the responsibility for demonstrating safety sits with you — speak to your local trading standards service or an accredited testing laboratory about leaching tests before your mugs go on a market stall.

Dust, gloves and a damp bench

Glaze ingredients are at their most hazardous as airborne dust, and dust is entirely a housekeeping problem. Keep these habits fixed:

• Wear an FFP2 or FFP3 mask whenever you weigh, sieve or mix powdered materials — and follow the mask manufacturer's fitting instructions.
• Work on a damp bench and wipe spills while they are still wet. Never dry-sweep or use a compressed-air blower.
• Keep lids on tubs and bags, and label everything. An unlabelled jar of grey powder is a hazard with no instructions.
• Use separate sponges and cloths for glazing and for cleaning the rest of the studio.
• Wear gloves for oxides and stains, which stain skin as readily as they stain clay.
• No eating, drinking or smoking in the glazing area, and wash your hands properly when you finish.
• Sand glazed surfaces wet, or with a damp cloth, never dry over an open bin.

Application checks at the bench

Most glaze faults are decided before the kiln is ever switched on. Run through these before a pot goes on the shelf:

1. Stir, then sieve. Settled ingredients give uneven results; lumps become pinholes and blisters.
2. Check the slurry consistency against your last good batch. A hydrometer, or simply weighing a fixed volume, keeps things repeatable.
3. Aim for an even coat. Thick enough to hide the clay, thin enough that texture still reads. The reliable test is weighing a test tile before and after dipping and comparing it with a tile you liked.
4. Wipe the foot ring clean, leaving a clear margin of bare clay where the pot meets the shelf.
5. Look for runs and pools at the base, under handles and inside rims, where glaze gathers.
6. Check the inside of every pot as carefully as the outside. Interiors are where thin patches and missed spots hide.
7. Let everything dry fully. Damp glaze and damp clay both cause trouble in a hot kiln.

Kiln loading precautions

Kiln wash is your first line of defence. Re-coat shelves when the coating flakes, and never load a shelf that is shedding — flakes stick to glaze and fired glaze sticks to shelves. For runny or crystalline glazes, use stilts or wads, and give anything that flows a wide berth from the shelf edge.

Leave space between glazed surfaces, including between a pot and the one beside it. Two glazed rims that touch will fuse. Keep the load stable so nothing topples as the kiln expands, and put a witness cone where you can see it through the peephole.

Ventilation matters as much as stacking. Firing drives off fumes from clays and glaze materials, so run extraction or open a vent, and never fire in a sealed room. If you work with lead-bearing glazes at all, keep them out of any kiln that also fires food ware; the cleanest answer is to avoid them entirely. And follow the kiln manufacturer's guidance on clearance, venting and what may safely go inside.

After the firing: what to look for

Unpack slowly. Check for crazing, shivering, pinholes, blisters and sharp edges, and hold each food-contact piece up to a bright light to inspect the interior. Test cutlery marking with the back of a spoon. If a pot fails any of these, don't sell it and don't drink from it — refire it, reglaze it, or keep it as a glaze test.

Then write it down. A photograph of the tile, the glaze name, the thickness and the firing schedule will save you repeating a mistake next month, and it will remind you which combinations earned a place on the kitchen shelf.

The ten-point check before you switch on

1. Ingredients read and safety data sheets filed.
2. Powder work done masked, on a damp bench, with tubs sealed afterwards.
3. Every pot sorted into food-contact or decorative.
4. Food pieces glazed inside with a liner you have fired and tested.
5. Glaze stirred, sieved and matched to a known batch.
6. Coat even, with no drips or thick pools.
7. Foot rings wiped clean.
8. Pots bone dry and free of loose glaze dust.
9. Shelves freshly kiln-washed, supports under anything runny.
10. Space between glazed surfaces, witness cone in place, ventilation running.

Work through that list out loud if it helps. It is quicker than chiselling a bowl off a shelf, and far quicker than explaining to a customer why the inside of their mug is cracking.

Photo: LUM3N / Pixabay