Sapstain
Bluestain in two month
old pallets
Although in its early stages sapstain (also called bluestain) has no strength reducing effect, pallet purchasers rarely like to receive new pallets or boxes like the one in the illustration. Also, since sapstain 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, but the vulnerable surfaces are those top deckboards that are in tight contact with the bottom boards of the pallet immediately above as clearly shown in the illustration. The worst stain patches in the illustration coincide with the pallet spacer blocks which took all the compression load of those pallets in the high pile above. This meant that even during a period of dry weather the high stack pressure kept a permanent 'local' high moisture level above and below the spacer blocks.
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. We understand the Forestry Commission sometimes use our Datasheet 81h (currently being revised) to assist them in responding to enquiries received on the frequent problem of bluestain in part-dried wood that has earlier 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.
Even well dried, close piled timber stored outside is vulnerable and we give an explanation of why and also the difference between HT and sterile timber (not the same) and how to avoid pitfalls using a range of protective measures in Datasheets currently being finalised.
For Downloading...
81e Kiln dried timber - development of moulds after drying - general (PDF 19kb)81h The causes of bluestain in packaging timber (PDF 50kb)81m Dry rot - does it apply to wood pallets and packaging (PDF 19kb)