Thermally-Modified Ash Wood

Thermally-Modified Ash Wood

Dark thermally-modified ash wood

Thermo ash is a thermally treated ash with a chocolate-coloured tinge throughout. For the thermal process the wood is heated up to 160°C (≈ 320°F) under deoxygenation for 24 h to 48 h. This increases the rot resistance of the wood. Also the capability for water absorption is reduced, which minimizes the risk of swelling and contraction. But a negative effect is the vulnerability towards longitudinal cracks, e.g. when drilling or screwing in. Note: Before using screws always remember to predrill!

Seat island RÜGEN, covered with thermo-ash

Seat isle RÜGEN, covered with thermally-modified ash wood

Oiling helps against wood turning grey

Without oiling, any wood will turn grey within a few months and take on the typical silvery appearance. Thermo ash has a typical life period of up to 20 years in a temperate climate. This is not significantly prolonged by oiling. The aim of oiling is to delay the greying of the wood.

As an oil we recommend (boiled) linseed oil which has been used for preserving wood for centuries. In order to produce boiled linseed oil the oil has to be cooked under deoxygenation. To ensure that the oil gets as deeply as possible into the wood we recommend to dilute the substance with turpentine oil and to apply it several times. The hardening of the oil is faster than with pure, cold-pressed linseed oil but can take a few days. Note: Always check that the oil has dried completely before using them for the first time!


Thermo ash is also often used for decking. Therefore the flat formats are particularly available. Thanks to our experience the following size is on stock: 25 x 120 mm² (≈ 9.84 x 47.24 in²)

With this standard size it is possible to obtain perfect seat rest covers with a total width of 500 mm (≈ 196.85 in), i.e. 4 x 120 mm (≈ 47.24 in + distance in between). Of course other sizes are still available.

Thermo ash seat rest

Thermally-modified ash wood in the standard size 25×120 mm² (≈ 9.84×47.24 in²)

Example and Mounting

When using thermally-modified ash wood the seat rest is screwed from below so that no screws are visible.

Storage Box with Seat Cover in Thermo Ash

No screws visible – ALVARO Storage box
with seat rest made of thermo ash

Surface and tension cracks

Even with thermally-modified wood surface cracks are normal and tension cracks possible. A normal width is 1-3 mm and the length can vary. Stress cracks mainly happen along the front, where the water intake and water release is higher. Note: Cracks are no reason for complaints.

Color difference and warping

Color differences are a natural characteristic of this natural product – because of the inhomogeneous structure and the different places of origin. Due to weathering thermo ash turns grey just like any other sort of wood. But this does not influence the durability. In order to preserve the color of the wood, it is recommended that the wood – if necessary – is treated once to twice a year with thermo-wood-oil. The oil closes the pores, so that dirt does not adhere and the later cleaning is simplified. As thermo ash is a natural product, color differences are possible and no reason for complaints.

We recommend

Holz is a natural product. This fact means that warping, color-differences, cracks, embeddings due to branches and other irregularities are possible. If you do not want these properties with your high-quality garden objects, we recommend to choose an industrially manufactured seat rest cover. We are happy to share our experience with you if you wish to have an alternative to wood. An alternative that guarantees perfect, constant, industrially manufactured quality.

Suggestion for tender template

“Thermally-modified ash wood, chamber-dried, residual moisture under 20%. Surface planed and sanded on all sides, all edges beveled 2 x 45°. Surface sliqutly oiled.”

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