Bit of microwave drying advice please

[OldWood]

Sorry guys - I know this one has been done to death, but I have a large amount of large wet wood (wych elm, cherry and oak) and I've found that the wych elm in particular has a stunning contrast between the white sapwood and the brown mature wood as long as it is not allowed to sit wet for too long, so I need to process it.

I've got a 12" roughed oak bowl going through the microwave at the moment and I'm taking it very slowly - it's got to lose an estimated 150gms in 1kg (28%RH down to 12%). I'm using defrost and started off with 2 minutes, increasing that to 4 as it dried to less than 20%, allowing it to cool completely before 'cooking' again.

The question is just how warm can I take the wood to. I'm not letting it get above warm (~26 C) at the moment - can I take it above that safely ? I've done a couple of small bowls successfully so far this way, but this big one is taking an age, hence the query to speed things up.

Thanks

Rob

 

[boysie39]

Rob ,it's your piece of timber and you have had good results the way you have done it before why risk losing a good blank .
If it were me stick to the tried and tested way . Slow is good if you get a result .

 

[OldWood]

Rob ,it's your piece of timber and you have had good results the way you have done it before why risk losing a good blank .
If it were me stick to the tried and tested way . Slow is good if you get a result .

You're absolutely right Eugene; but, on the basis that chemical processes (yes I know this is more physics than chemistry, but the same should apply!) do in general double in speed for every 10C, then if someone said that they found success running the wood to say 35C it would just about halve the time it's currently taking me. That is the logic of me asking the question.

If there is no consensus, then I will try a gash piece and see what happens.

Rob

 

[dickm]

I think part of the answer is in your last post. The drying part of this is a physical process of moisture movement, so it won't increase in rate as fast as you suggest, but in direct proportion to the temperature difference you can obtain between the moisture in the wood and that in the air.
But A-level physics was a LONG time ago, so that may be rubbish

 

[tekno.mage]

Don't rush your microwave drying - you'll end up with splits in the wood. Carry on with what you are doing (slowly & gently) and you might want to "rest" the wood out of the microwave by wrapping it in layers of newspaper and allowing it to cool slowly between bursts (the newspaper help keeps the damp in a bit longer so the outside of the wood doesn't dry too quickly and split). I've found that this slow and gentle process works best, especially for very wet wood - but even this method will not garentee there will be no splits :-( Another trick that works for bigger items is to paint the endgrain parts of the roughed bowl with PVA glue after microwaving so that this part doesn't dry out quicker than the long grain parts.

Good luck with it.

 

[OldWood]

Don't rush your microwave drying - you'll end up with splits in the wood. Carry on with what you are doing (slowly & gently) and you might want to "rest" the wood out of the microwave by wrapping it in layers of newspaper and allowing it to cool slowly between bursts (the newspaper help keeps the damp in a bit longer so the outside of the wood doesn't dry too quickly and split). I've found that this slow and gentle process works best, especially for very wet wood - but even this method will not garentee there will be no splits :-( Another trick that works for bigger items is to paint the endgrain parts of the roughed bowl with PVA glue after microwaving so that this part doesn't dry out quicker than the long grain parts.

Good luck with it.

Hi Kym - many thanks for that; I appreciate your advice. Interesting that you've suffered splits even with care. Going back to my initial enquiry, have you ever measured the wood temperature just after 'cooking' it ?

And carrying on with the questions - are you heating something that is down to final size or at the thick cut stage (marmalade ? :roll: ) And carrying on the question theme, what EMC do you reckon on getting down to ?

All this started from a requirement to dry some 3 yr felled but water wet oak from a friend's garden - he bought the house from previous friends who had planted the oak in memory of a son who died in an accident. The tree had to be felled and a bowl from it was thought an appropriate gift, but not in 5 years time. The bowl is 250mm dia with 15mm thick walls currently. My cheap (EBay) RH meter seems to be quite accurate from plotting the weight change I'm getting and I'm reckoning on getting it down from 28 to 12% which I'm calculating as 1065gms down to 930.

As an aside, I did a shallow open bowl, several rounds of zapping it, and then only pressed the power button once instead of twice to get to Defrost so it got 3 minutes of High !! :shock: It warped rather than split; fortunately it was turned nearly to final shape so with a LOT of sanding later, it's a nice bowl that cannot be repeated !

Oh - and how do you PVA the end grain of a bowl ?

Rob

 

[tekno.mage]

Hi Rob,

I've never actually measured the temperature of the wood after cooking it - it comes out quite warm to the touch (but not hot enough to burn fingers whilst wrapping in newspaper) and steaming very slightly, It stays warm for quite a while after (around 10-15mins) and the newspaper gets damp with the steam. I microwave wood at the thick cut roughed out stage, rather than the finished size stage. I only ever microwave on defrost setting or lower (lower setting if the wood if quite thick and very wet.) and only for 2-3 minutes at a time.

In terms of EMC, I don't know - it's not how I do it. I simply use weight to tell if the wood is dry - weigh it every day (in doors, not in the shed as the ambient humidity is usually lower in the house) and when the weight stops falling and stays constant the piece is as dry as I can make it without resorting to the "kiln" of the airing cupboard. When I make boxes, I "kiln" dry my roughed boxes by leaving in them in the warm airing cupboard for several months and again weighing them periodically to see when the weight levels off. This process overdries the wood (you can tell because when taken out of the airing cupboard the piece starts to gain a few grams in weight particularly on wet days!)

Regarding applying PVA to the end grain of a roughed bowl , I simply paint on cheap PVA glue with a brush to the end grain areas both inside & outside of the bowl. This dries and semi-seals the endgrain (PVA glue is slightly permeable) to equalise the drying of the wood. The PVA does not penetrate very far and will be turned away when you finish the bowl.

I too have made the odd microwaving misake with the full power option! On thin work (the walls must be very thin and the same thickness all over the piece) you can use this method to deliberately distort the piece with your hands on removal from the microwave - which can be quite interesting and works because hte steam has "plasticised" the wood (like with steam bending). However, usually I make this mistake with thicker bits - and the results are either splitting or in extreme cases charring :-(

 

[OldWood]

Kym - many thanks indeed for replying so fully.

I guess I'm going in the right direction even if it is slow - I do suspect that you are heating to a little warmer than I and might experiment a bit more with going up a few degrees at a time. I like the idea of the warm cupboard - I have such a cupboard above the hot water tank, originally built for fermenting wine (!), and that could now be put into use for wood. I also like the idea of the deliberate distorting.

I got some heavy branches from a fallen wych elm in the winter - these have been split for bowl blanks, but I became concerned recently about the sap wood loosing it's lovely whiteness, and am in the process of wet turning them and putting them to dry, so the 'airing' cupboard will do nicely.

Again many thanks
Rob