Warping

Why does Stabilised wood sometimes warp?

Properly stabilised wood warps for almost only one reason and one reason alone, Interfibrillar tension. This term is not really used much in the timber industry, it’s more medical, but in the true sense of the word it is “the forces that exist between individual cellulose microfibrils within the structure of the wood.

Stable wood starts right at the beginning of the trees life, in an ideal world a timber tree would need to be planted in suitable conditions with proper care and attention. This is rarely the case however, for most suppliers the work really starts when it is first cut. As soon as the tree is felled the clock is ticking, for the best results the timber should be left as a whole trunk with the ends sealed and bark removed and air dried for decades, unfortunately this is not commercially viable. The next best option is milling into slabs and air drying to <20% MC (Moisture Content), this allows the wood to dry out slowly and lose all of its free water and some of its bound water.

Free water lives in the hollow cell cavities of the wood (the lumens) and can quickly evaporate without causing the wood to shrink or crack. Bound water, on the other hand, is chemically bonded to the cell walls. When the wood dries, the free water evaporates first, followed by the bound water (Which takes much longer to evaporate).

In wet wood, the water molecules are bound to the hydroxyl groups of cellulose, hemicellulose, and lignin through hydrogen bonds especially when below the fibre saturation point (~30%MC). As the wood dries out, these water molecules are evaporated, disrupting the hydrogen bonds between water and the wood. This allows the fibre chains to move closer together (Adding tension into the wood). Once the water molecules have been removed, the wood fibres are left with their hydroxyl groups and other electron-rich regions.

Drying will always add Interfibrillar tension to the wood, in thicker pieces it is rarely a problem after stabilising but it is the reason wet wood will warp and crack whilst drying. The water acts as a "plasticiser" within the wood, which softens the cell walls and allows for easier deformation. As water is removed, the wood loses this plasticising effect, resulting in a stiffer and more rigid structure (and therefore more prone to splitting).

Once the wood is under 20% MC most of the hard work is done, it is at this point I can take it to to a vacuum kiln. The vacuum kiln is a brilliant piece of equipment, the vacuum lowers the boiling point of any remaining bound moisture and the heat drives it off with incredibly minimal cracking or warping. Whilst it does come with drawbacks if it is done correctly it will save a huge amount of time and money.

This now leaves blocks that are ready for stabilising, that are moisture free, split free, and incredibly stable.

Once stabilised, the wood is caught in an almost freeze-frame of the state in which it went in to the chambers. If it was full of tension it is highly likely to warp, if not then this is unlikely to happen. In 20mm+ thick blocks it really isn’t too much of an issue but when scales are being cut into thinner slices then these may be more prone to warping.

To put it simply, stabilised scales that are warping is usually due to the release of Interfibrillar tension within the pieces of wood.

Can it be fixed?

The short, and unhelpful, answer is ‘sort of’. As mentioned above, stabilising effectively freezes the block in its current state, if there is lots of tension beforehand it will warp. Once the piece has warped there are a few things that can be done.

The best and most effective method I have found is using heat and applying pressure:

Heat the warped piece in either a conventional kitchen oven or in a sealed bag in a temperature controlled water bath to 100°C for 30 minutes, this will take the resin above its ‘High Glass Transition Temperature’ (TG point). This makes the resin slightly plasticised and therefore malleable, Once at temperature, place it in a vise (ideally a wood vise or something with full contact of the scales) and tighten it up.

If the warping is minor, you can do it in one heating cycle, but for major warps it may take multiple heat cycles and clamps. It needs to be noted that burr pieces especially may crack during this process due to the pressure placed on the piece with weakened grain directions.

The resin I use contains thermosetting polymers, this means that once it is cured it cannot be re-melted. It has a TG point of around 100°C, this is the point where the resin transitions from a hard, glassy state to a more flexible, rubbery state. This is what allows it to be somewhat malleable whilst heated.

Finally, leave it to cool under pressure for 24-hours and hopefully it should have rectified the issue, This isn’t a magic fix for everything, just something that can be a very useful tool in the arsenal.

How can I mitigate these issues in future?

The best way to ensure you are getting high quality blocks is to use reputable suppliers, who know what they are doing, know their whole supply-chain and really understand why it is happening. Many suppliers will be happy to chat about all of these things and it is a great way to sort quality suppliers from the rest.

Whilst I do my best to understand every step that my local and overseas suppliers undertake to produce high quality timber it is not always possible, this means that some blocks can still have interfibrillar tension that won’t be released until cut.

THE BEST WAY TO PREVENT WARPING AFTER YOU CUT YOUR OWN SCALES IS TO CLAMP THEM IN A VISE, I cannot stress this enough. When you cut a block into scales it can start to warp within 5 minutes, if you clamp it up as soon as you cut it and leave it under pressure for 24-hours the interfibrillar tension will be released and the piece will be unlikely to warp. I have done this with over 3000 sets of scales over the last year or so, and after clamping they always stay as straight as a die for months after.

I tried to stabilise my own wood and it warped! What happened?

The main two reasons it will warp are either it is wet, or has been wet. If your blocks are improperly dried and you try to stabilise them they will likely warp during the cure, this rapid temperature change causes the remaining moisture in the block to escape and therefore warp the wood. Whilst this does have implications in the quality of stabilising, this usually isn’t too detrimental to the final piece, provided it was at a low moisture content to begin with.

The other main issue is caused by improper stabilisation. When done correctly, the resin saturates all of the fibres and fills all of the empty spaces in the cells with resin, if it is treated incorrectly this will leave fibres not fully saturated and gaps will remain in the lumina. Over time the fibres can reabsorb moisture and then swell up, this is what will lead to cracking and warping later down the line.

The most important thing to understand while treating your raw wood is to make sure it is completely dry before it goes into the resin, this means cooling it in a vacuum to stop moisture resorption and allowing the wood to come to its hygroscopic equilibrium. Proper stabilisation is then key to ensure good results too, there are loads of great tutorials out there explaining the process.

https://www.youtube.com/watch?v=DdaBfscf2IY

This is a link to a video I did with Vinz from UKbladeshow explaining my processes back in early 2022