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Drying the tiles can be one of the most problematic processes to get right. Due to the nature of the project which uses small diameter logs, the pith of the tree stays in the centre of the tile. This configuration of the timber makes it exceptionally prone to splitting from the circumference edge of the tile to the pith. This is due to the stresses that occur in the tile if dried too quickly. Some species are more problematic than others with the worst being oak and beech. The project looked at two forms of drying. The first was using a Heat and Vent Kiln and the second was a Heat Treatment Oven. The requirement of both was to get the moisture content of the tiles down to between 8% and 10%, which is required for modern indoor timber flooring with as few losses through tangential splitting of the endgrain tile as possible. A moisture metre was used to measure moisture content.

When drying using a kiln, the tiles needed to be air dried to below 20% mc. Due to the nature of the tile with its two faces being endgrain, they have the ability to dry very quickly. It is preferable if the drying tiles are kept out of direct sunlight and not exposed to excessive air movement. Oak and beech are particularly vulnerable. These were then placed on special racks, which are the key to preventing tangential splitting, before being placed in the kiln. On average a one metre cube oven can accommodate1300 blank tiles (115mm x 115mm x 15mm). The Heat and Vent kiln runs by heating the internal space of the box up to 30 degrees C and circulating the air around and through the timber with a series of fans.

Detailed information on the kiln.

When the kiln was properly stacked and the temperature and relative humidity were set to correspond with the required moisture content of the dry tiles, the average time for drying was 10 days from a baseline of 20% moisture content.

A one metre cube capacity Heat Treatment oven was purchased to trial the drying of endgrain tiles in 2010. This oven heats up to 190°C with a continual supply of steam being pumped in and circulated around and through the timber stack continually. It is a more expensive process than kilning due to the amount of electricity it uses to heat up the chamber. However this is offset by the short drying times of, on average, ten hours. The green tile blanks were stacked onto galvanised steel racks. It is imperative to take the timber through the primary process of cutting the billets to cross cutting and then placing on the purpose made drying racks as quickly as possible. Delays can cause end checking of the billets, spalting of the cross cut tile blanks and radial splitting.

The temperature is taken up to 120°C and held for one hour before it is increased to 190°C where it is held for a few hours. The longer the temperature is held at 190°C the darker the tiles become.

The noted benefits from this process were:

  1. The ability to darken the tiles down in stages to a rich mocha colouration. The tiles can appear like teak or mahogany.
  2. The timber is unable to re-absorb moisture after treatment as it would prior to treatment, making it less prone to movement which is of great benefit to timbers like beech.
  3. An improvement in machinability and finish.
  4. A quick turn around of stock.
  5. As with the kiln drying, the drying racks that have been developed are key to having low wastage through tangential splitting of no more than 7%. However this did go up to around 15% with oak and beech.