Freezing (or heating) borer out of wooden items

Does freezing really kill borer?

Freezing really does kill borer larvae, pupae and eggs if the cold reaches the middle of the wood and stays there long enough. Conservation institutes use low-temperature treatment on museum collections because it works without chemicals. NZFFA also lists freezing among the alternative treatments for common house borer. The weak point is time: thick timber takes hours to cool right through.

How do you freeze a wooden item properly?

Freeze a wooden item by sealing it in a plastic bag with as much air pressed out as you can, putting it in a freezer that holds about −20°C or colder, and leaving it at least a week. Then let it warm back to room temperature while still sealed, so condensation forms on the bag, not on the wood. Unwrap it only once it's fully warmed.

1. Clean off dust so you can spot new activity later.
2. Bag and seal the item; add a second bag for valuable pieces.
3. Freeze for a week at about −20°C. Domestic freezers usually run a little warmer, around −18°C, so allow longer.
4. Thaw sealed for at least a day, until the bag is at room temperature.
5. Watch it for new dust over the next flight season.

What can go wrong when freezing furniture?

Freezing can go wrong in three ways: the cold doesn't reach the core because the treatment is too short, condensation wets the wood when an unbagged piece warms up, or delicate materials crack. Sealing the bag and thawing it closed deals with condensation. Mixed-material and finely veneered pieces are the ones to take to a conservator.

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Freezing also gives no lasting protection. If the piece goes back into a house with active borer, the next generation of beetles can lay eggs in it again. That's why a residual treatment on its bare back and undersides is worth adding afterwards, as our furniture guide explains.

Can heat kill borer in furniture?

Heat can kill borer, and controlled heat treatment is used commercially and in some museums, but it's riskier at home. Wood needs to be heated right through, which can soften old animal glues, lift veneers, crack timber as it dries and spoil finishes. A hot car or a sunny window isn't a controlled treatment. Freezing is the safer home method for small pieces.

Leaving a piece in strong sun on a summer day won't reliably kill larvae deep in the wood, and it can damage the piece. Our answer on natural borer treatment looks at other non-chemical ideas and how well they hold up.

When is a liquid treatment better?

A liquid treatment is better when the item is too big for a freezer, when it's part of the house, or when you want lasting protection. Injecting flight holes and coating bare timber kills larvae in the piece and leaves a layer that stops new eggs. For anything built into the house, freezing isn't an option at all. The flight-hole injection page covers the method.