If your packaging carries an HT stamp, it has been through a heat treatment chamber. It is a simple process to describe and a surprisingly exacting one to run correctly, because the standard it satisfies is written in terms of what happens in the middle of a piece of timber, not what the chamber's air temperature says.
The requirement
ISPM 15 sets one number that matters: the core of the wood must reach 56 °C and stay at or above it for 30 continuous minutes, throughout the entire profile of the timber. Not the surface, not the average, and not for a cumulative 30 minutes across the cycle — 30 unbroken minutes at the coldest point inside the thickest piece in the load.
That is why a treatment cycle takes hours rather than half an hour. The chamber has to drive heat from the outside of each board to its centre, and wood is a good insulator. A thick block for a pallet foot takes considerably longer to come up to core temperature than a 22 mm deck board, and the cycle is dimensioned around the worst case in the load.
An alternative schedule, dielectric heating, reaches 60 °C for one continuous minute throughout the profile. It works by heating the wood volumetrically rather than from the surface inwards, which is why the time is so much shorter. It is used at scale for some product types, but conventional chamber heat treatment is what most European packaging producers operate.
How a cycle actually runs
In practice the operator loads the chamber so that air can circulate around every stack, places temperature probes into the core of the largest cross-sections and in the coldest parts of the load, and runs the cycle until the probes confirm that the schedule has been met. The recording is the point: without a temperature log from probes in the wood, there is no evidence that the treatment happened, and the mark that follows would be meaningless.
Loading matters more than people expect. A tightly packed chamber with blocked airflow will show a perfectly good air temperature while the middle of the load lags far behind. This is one of the reasons treatment is worth having done by the producer of the packaging rather than bolted on afterwards by whoever has a chamber free.
Treatment is not drying
Heat treatment and kiln drying often use the same equipment, which is why they get conflated. They are different objectives:
- Heat treatment is a phytosanitary measure. It targets a defined time and temperature to kill insects and nematodes. It has no defined endpoint for moisture content.
- Kiln drying targets a moisture content — typically expressed as a percentage of oven-dry weight — to make the wood dimensionally stable and much less hospitable to mould.
A heat-treated board can still leave the chamber wet. In fact, a short HT cycle on green timber will drive moisture to the surface and, if that packaging is then wrapped in film and containerised, you have created ideal conditions for surface mould. If your product is sensitive to staining or your route is long and humid, specify a moisture content in your order, separately from the HT requirement. It is a different line item and often a different price.
What happened to fumigation
The other treatment codes you may see are MB, methyl bromide, and SF, sulphuryl fluoride. Methyl bromide was for many years the default alternative to heat, and it is still encountered on packaging arriving from some regions. It is an ozone-depleting substance and has been withdrawn from use in the European Union, so European producers do not offer it.
For a buyer, this mostly matters in one direction: if incoming packaging arrives marked MB, that tells you something about where it was produced, and some consignees have their own policies about accepting it. For outbound European shipments, HT is the practical answer.
Debarking, and the detail that trips people up
Treatment alone is not enough. The wood must also be debarked. The standard tolerates small residual pieces of bark: less than 3 cm wide regardless of length, or, if wider than that, an individual piece with a surface area under 50 cm². Beyond that, the bark has to come off.
This is a genuine rejection cause in the field, and it is easy to miss during a visual check of a finished pallet because the offending strip is usually on the underside of a bottom board where nobody looks.
Does heat treatment change the wood?
At ISPM 15 schedules, only slightly. The temperatures involved are well below those used for thermal modification of timber, where wood is taken to 180–230 °C to change its properties permanently. At 56 °C the wood darkens marginally at most, loses some moisture, and its strength is essentially unaffected. You should not expect treated packaging to look or behave differently from untreated packaging — which is precisely why the mark exists, since you cannot tell by eye.
Practical points for buyers
- Ask whether treatment is done in-house. In-house treatment means the producer controls loading, probe placement and records, and it usually means shorter lead times than sending finished packaging out for treatment.
- Ask for the registration number under which the mark is applied.
- If mould or staining would be a problem, specify moisture content separately — HT alone does not guarantee dry wood.
- Remember that treated, unaltered packaging keeps its mark for its whole service life, so a returnable crate does not need re-treating on every trip.
We run heat treatment at our own site in Judrėnai, which means treatment and marking are part of the production schedule rather than a separate stage bolted on at the end.
