How do I use casting resin safely?

Resin is unforgiving in two directions at once. It punishes imprecision — a slightly wrong ratio or unmixed material on the cup wall produces a piece that stays sticky permanently — and it punishes casual handling, because the sensitisation it causes is cumulative and does not announce itself until it is established.

Safety

What you'll need

• Nitrile gloves and long sleeves
• Digital scales or graduated cups, matched to the stated ratio
• Two mixing cups and flat stirring sticks
• Ventilation — a window and an extractor, or work outdoors
• A silicone mould and a level surface
• A heat source for surface bubbles, used briefly (optional)

Step by step

1. Set up the room before you open anything.Ventilation running, a covered work surface, gloves on, a level place for the mould to cure undisturbed, and everything you will need within reach. Once the resin is mixed you are on a clock and you cannot leave to fetch something.
2. Check whether the ratio is by weight or by volume.This is the single most common cause of a batch that never cures. The same product has different numbers for the two methods, and mixing by weight to a volume ratio produces a mixture that is chemically wrong. Read the label and use the matching tool.
3. Measure precisely, not approximately.Epoxy is a chemical reaction between two parts in a fixed proportion. Excess of either part does not cure and remains as a sticky component through the piece. There is no eyeballing this.
4. Stir slowly, scraping the sides and the bottom corner.For the full time the manufacturer states, usually longer than feels necessary. Slow stirring folds in far less air. Unmixed resin clinging to the wall of the cup is the classic cause of sticky patches, and it does not cure late — it stays sticky forever.
5. Transfer to a second clean cup and stir again briefly.This leaves the film of possibly unmixed material from the first cup behind. It is a professional habit that costs one extra cup and eliminates most cure failures.
6. Let it stand before pouring.A few minutes on the bench lets the larger bubbles rise and break. This is more effective than trying to remove them once poured.
7. Pour in a thin stream from a low height, in layers.A thin stream breaks bubbles as it falls. Every resin has a maximum pour depth, because the curing reaction generates its own heat and a deep pour can overheat, yellow, crack or in extreme cases smoke. Deep-pour resins exist specifically for thicker work.
8. Deal with bubbles at the surface only.A brief pass with a heat source over the surface pops surface bubbles. Keep it moving and keep it away from the mould. A cocktail stick releases bubbles clinging to the mould wall and in undercuts.
9. Seal porous inclusions before casting them.Wood, dried flowers, paper and fabric all hold air that comes out into the resin as a stream of bubbles, and organic material can also discolour. Seal with a thin coat of resin or a spray sealer and let it cure first.
10. Leave it completely alone while it cures.Dust-free, level, at the temperature the manufacturer specifies, for the full stated time — often a day to demould and considerably longer to reach full hardness. Moving or touching it mid-cure leaves permanent marks.
11. Dispose of waste properly.Cured resin is inert and goes in general waste. Uncured resin, contaminated cups, sticks and gloves are hazardous waste for a recycling centre. Never pour resin down a drain and never let any of it near food surfaces.

Tips

• Work between about eighteen and twenty-five degrees unless the product says otherwise. Cold resin is thick, holds bubbles and cures slowly or not at all; hot resin cures too fast and can overheat.
• Warming the sealed bottles in warm water before mixing makes the resin runnier and releases bubbles far more readily. Warm the bottles, not the mixed resin.
• Silicone moulds release without any release agent and are worth the cost. Anything else usually needs a release agent and often ruins the piece coming out.

Common mistakes

• Using latex gloves — Epoxy passes through latex. Nitrile is the correct material, and the reason it matters is that sensitisation is cumulative — the exposures that end someone's ability to use resin were individually trivial.
• Cleaning resin off skin with acetone or alcohol — Solvent thins the resin and carries it into the skin rather than removing it. Soap and warm water, promptly, is the correct method.
• Pouring deeper than the product allows — The curing reaction is exothermic and a thick mass cannot shed the heat it generates. It yellows, cracks, warps the mould, and in a large enough mass can smoke or set fire to a container.
• Working in a bedroom or a closed room — Uncured resin vapour accumulates in a still room and it is a respiratory sensitiser as well as a skin one. Curing pieces continue to off-gas, so the room they cure in matters as much as the one you mix in.

If it doesn't work

Questions people ask