Bluestain
Bluestain in two month
old pallets
Although in its early stages bluestain (also called sapstain) has no strength reducing effect, pallet purchasers rarely like to receive new pallets or boxes like the one in the illustration. Also, since bluestain is often accompanied by a musty smell, pallets to be used for food or drink may actually be rejected by customers. The illustration shows a typical attack in a pine species such as Scots pine or French maritime pine which are known for developing bluestain in warm weather when moisture is trapped between close piled boards. Banded packs of ready cut boards are particularly likely to be affected since almost every finished timber surface is in contact with another and will exclude the air and retain the moisture - a recipe for bluestain at all times except when the air temperature is very cold.
Assembled stacked pallets have many open surfaces allowing good ventilation and rapid moisture loss allowing ready drying. However, there are vulnerable surfaces on the outside of each pallet. These are the top deckboards that are in tight contact with the bottom deckboards of the pallet immediately above, as clearly shown in the illustration. The worst bluestain patches in the illustration coincide with the positions of the pallet spacer blocks which take all the compression load of those pallets in the pile above. This means that even during a period of dry weather, permanent 'local' high moisture exists which may cause bluestain.
The spruces also suffer from bluestain so users of all species need to be aware of how the trouble starts and how to avoid it, moisture ignored is often the culprit. We have many enquiries from members on the subject and as a result have built up a range of Datasheets on the subject to help avoidance. The Forestry Commission sometimes send enquirers our Datasheet 81h in responding to enquiries received on the frequent problem of bluestain in part-dried wood that has been heat treated (HT) under the UK Phytosanitary Marking Programme. HT timber is frequently not dried down to the critical 20% moisture content that is known as the 'decay safety line' because that is not part of the ISPM 15 requirement.
Well dried, close piled timber stored outside is vulnerable and we give an explanation of why, the difference between HT and sterile timber and how to avoid pitfalls - in various Datasheets on this website.
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75e Wood Equilibrium Moisture Content Table (EMC) (PDF 45kb)81a New timber pallets and packaging - hygiene issues (PDF 20kb)81b Used timber pallets and packaing - hygiene issues (PDF 20kb)81d Is bluestain bad for health - hygiene issues (PDF 24kb)81g Pin moulds - development of moulds after kiln drying (PDF 99kb)81h The causes of bluestain in packaging timber (PDF 82kb)81j Issues with mould and stain appearing after kiln drying of pallets (PDF 20kb)81m Dry rot - does it apply to wood pallets and packaging (PDF 19kb)87e Palletised concrete products - tannin staining by hardwoods (PDF 28kb)