Moisture Content

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Moisture Content

Postby JermyB » Tue Jan 29, 2013 10:32 pm

I am having some dilemmas regarding moisture content..maybe some people have some experience to offer.

When I am assembling a mortice and tenon joint then I understand the best combination is a 100% dry tenon and a mortice with a moisture content around 20-30% (when working with Ash).

For the 100% dry tenon this is easy...either I A) leave it for ages or B) try and speed things up with artificial drying whilst constantly weighing the part waiting for a consistent weight.

For the 20-30% moisture content of the mortice I am find this very difficult. The wood that I use comes from various sources and I'm never sure how long it has been felled for. Due to this when I start with the part I have no idea of the starting moisture content. Once I have processed the part 'green' I then need to leave it for a period of time to achieve the desired moisture content. This period of time will obviously depend heavily on many factors (volume, species, humidity, temperature etc etc). It is difficult to use the weighing method as I don't know what either the fully green weight was or the full dry weight will be in order to calculate a 20% weight.

A possible answer to the problem would be a moisture meter but I have the following issues with this option: -
> The cost effective moisture meters measure the content across two pins and therefore can only really tell you the moisture content of the wood surface not the wood inside.
> The meters that 'look inside' the wood are considerably more expensive but would possible give me a better idea of what is happening inside a post.

I was wondering what other people do...guesswork, science, technology?
You can always take more off...
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Re: Moisture Content

Postby toscano » Tue Jan 29, 2013 11:11 pm

I think this is over-thinking a relatively simple concept:
A tenon that is dry (that is, drier than the moisture equilibrium it will reach at its permanent place (your dining room, if it's a chair, for example) will expand as moisture is reabsorbed.
As long as the corresponding mortice is NOT drier than said equilibrium (so, not bone dry), this method will work.

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Re: Moisture Content

Postby gavin » Sun Feb 03, 2013 12:30 pm

JermyB wrote:I am having some dilemmas regarding moisture content..maybe some people have some experience to offer.

Your own experience will be the best teacher by far.

JermyB wrote:I was wondering what other people do...guesswork, science, technology?

They just do it. Whack together some test pieces yourself. See how you get on. That I find a tenon 14.0 mm to 14.5 horizontal and 15.2 mm vertical will happily drive into a 13.9 mm diameter hole will be irrelevant to someone without a vernier gauge. Durable furniture was made long before Vernier gauges, so I suggest you just do it too. If you are analytic and you have a vernier gauge, digital scales and an oven you could have fun determining the moisture contents of your various components. I suggest you'll be more productive and have more fun if you just make furniture - or a series of test pieces. What books have you as references? You may need to consider the effect of grain alignment for the variation in ambient humidity conditions of your particular climate.
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Re: Moisture Content

Postby robin wood » Sun Feb 03, 2013 12:53 pm

toscano wrote:I think this is over-thinking a relatively simple concept:
A tenon that is dry (that is, drier than the moisture equilibrium it will reach at its permanent place (your dining room, if it's a chair, for example) will expand as moisture is reabsorbed.
As long as the corresponding mortice is NOT drier than said equilibrium (so, not bone dry), this method will work.

best,
-toscano


This is good advice as is Gavin's just make some up. Sounds like you have been reading Mike's writings. He didn't learn by reading he learned by making lots. 20% moisture is just a way of saying it's not as important for the mortice to be dry as it is for the tenon to be dry.
As for the starting moisture content, whether it has been felled 3 days or three months ago it will have similar moisture content if it is still in the round. It only really starts to dry when you cleave and work it.
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Re: Moisture Content

Postby Shankar » Sun Feb 03, 2013 6:45 pm

Before commenting must say I don't know anything about moisture content.

But logically if you are weighing the piece of wood you are putting the tenon on till dry then you can work out the original moisture content by weighing before and after drying. As long as you are using the same wood for the mortise you will know the approximate moisture content. Or cut off a piece of the wood weigh before and after drying (if the piece of wood is going to dry out while being worked.)

Weight change divided by original weight times 100 = moisture content of original.

Or am I missing something?

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Re: Moisture Content

Postby Brian Williamson » Sun Feb 03, 2013 9:08 pm

Shankar wrote:
Weight change divided by original weight times 100 = moisture content of original.

Or am I missing something?

Shankar


For most things, Shankar, you're not. However, moisture content in wood is slightly peculiar. It is measured as a percentage of the dry weight of the wood.

So, if a piece of wood weighs 200gms and the weight comes down to 100gms after you've driven off all of the moisture, then the original moisture content of the wood was 100%.

Equally, if a piece of wood weighs 1 1/2 lbs and the weight comes down to 1lb after you've driven off all of the moisture, then the original moisture content of the wood was 50%.

Standing timber can have moisture contents of between (about) 60% and 120%.

Wood kept outside but under cover (ie firewood) should come down to around 15%.

Wood brought inside should come down to about 10%/12%. Turn the central heating on and you can get it down to around 6% - 8%. Given enough time.

However, moisture loss and shrinkage do not go together hand-in-hand. Moisture is lost in two phases. Firstly you lose the 'free' moisture (ie, the moisture in the centre of the cells). This does not induce shrinkage. Secondly you start losing the moisture in the cell walls, and this is when shrinkage starts to happen.

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Re: Moisture Content

Postby ToneWood » Mon Feb 04, 2013 8:44 pm

Brian Williamson wrote:However, moisture loss and shrinkage do not go together hand-in-hand. Moisture is lost in two phases. Firstly you lose the 'free' moisture (ie, the moisture in the centre of the cells). This does not induce shrinkage. Secondly you start losing the moisture in the cell walls, and this is when shrinkage starts to happen.
...
Interesting.

I'm not an expert on this but I've recently dealt with this a few times. I try to work all of the wood green (wet), when practical. Then dry the tenon parts fairly quickly indoors (utility room for slowish dry and/or near wood-burner for fast dry...can cause cracking though) and leave the morticed parts out in the shed - it's been so damp this year that things left in the shed/garage don't seem to dry at all :(. I shaped the tenon parts but leave them too large to fit the mortices - with the expectation that I can fine-tune their size after drying. The first time I did this, the tenon parts (legs) did not need any further reduction after drying - which is "cutting it a bit too fine" IMHO; however, the joints are still a good tight fit, so I think I may be ok :).

I think the basic idea is that the tenons be dry as possible then fitted tightly into the mortice. The mortice can then shrink down even tighter onto the now dry/stable tenon. Also, fresh ash (for example) seems to be somewhat compressible which may also help (or is that just my imagination).
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