Thread: You killed it - Now what??? Basic steps in timber drying for a quality product

Another fascinating thread John. A couple of questions for you...

I'm curious as to what kind of lead time is involved from stacking to market, particularly for the eucalypts. Do you air dry the boards till 20%mc then kd to 12 or whatever to finish them off? I know in my own little micro attempts at milling that the "inch per year" guide for air drying seems to hold true. What I cut last year is generally usable the following but typically lies around longer.

And what about larger dimensioned lumber like square posts? I'm thinking 200x200mm or larger. Not that there's a demand for bridge or pier lumber these days but how would you prepare that kind of sized timber for market and how long would it take?

Originally Posted by Canoath

Another fascinating thread John. A couple of questions for you...

I'm curious as to what kind of lead time is involved from stacking to market, particularly for the eucalypts. Do you air dry the boards till 20%mc then kd to 12 or whatever to finish them off? I know in my own little micro attempts at milling that the "inch per year" guide for air drying seems to hold true. What I cut last year is generally usable the following but typically lies around longer.

And what about larger dimensioned lumber like square posts? I'm thinking 200x200mm or larger. Not that there's a demand for bridge or pier lumber these days but how would you prepare that kind of sized timber for market and how long would it take?

The year per inch of thickness is a pretty ordinary rule of thumb. Which isnt to say that its bad because its not, its just not real good either. Theres too many variables come into it - species... thickness (because drying time is exponential to thickness, a 50mm thick section doesnt take twice as long to dry as a 25mm section, its more like three times as long), and location. Location factors in both location in a general sense because your timber in the Riverena would dry a lot faster then mine does here due to drier climate; but also location in a "how much the wind can get at it" kind of way inside those broad geographic areas.
Species being a broad "eucalypt"can narrow it down a bit but not a whole heap: theres a lot of difference between dryinjg flooded gum or Tassie Oak with a density range around 700kg/cubic meter and say Ironbark or Forest Red Gum with a density around 1200 kg/cubic meter. Anyhowm inch per year of thickness is the rule of thumb and more or less it might be right, give or take 6 months.

I air dry stuff as long as I possibly can. EMC for us here is around 15%, everywhere we try and send it seems to want 10% or less. Mostly I work on around 90 days as a minimum before going in the kiln, jsut to get it below 20% because running the kiln to suck free water out is costly. Because our EMC is relatively high I try not to run stuff through the kiln until its part of an order though thats not always possible: thats partly so I can try and meet moisture specs according to destination, and partly because its hard to hold timber dry for long in this climate.Post kiln we do the right thing regarding prompt packaging to keep the air away from the freshly dried boards but it can be very difficult: Hot dry timber leaving the kiln can suck up moisture like a sponge, and all it takes is a decent tear in the wrapping to go unnoticed and next thing you know I've got a customer unhappy because the wood doesnt arrive at the right moisture content.

Larger dimension timber is problematic to impossible. Really big sections or whole logs never dry out fully, or we're talking decades or centuries maybe. 200 x 200's and larger are mostly sold as green off saw for construction purposes or as graded timber. (To carry a structural grade its only got to be at 20% or less, which doesnt take long.) I play around with drying stuff 100 to 150mm at times, but its not really possible to get a consistent result without either vacuum or RF at that thickness. I usually air dry for a year then toss some in the box with another load when theres space... dry them a bit, pull them out and wrap them... let them rest so moisture can draw from the inside of the board to the now dry edges... toss them in again a month later... repeat over and over.
I calculated once that it would take 175 days for my kiln to dry a load of 100mm from 20% to 10%: theres neither enough price premium or demand in the thicker sections to make that economical. What I do is better then nothing but how much better is hard to say: I've had 150 squares on the same skid as 200 x 100's and the 150's were dry inside when tested but we ripped some of the 200x100's in half and they hadn't dried much at all. I suspect grain orientation as the cause, because that does effect drying time also.

I'd love a Vac kiln: if I ever win lotto... (Mary says to me "what would you do if you won the lottery?" My answer..."keep sawmilling till its all gone"

Originally Posted by John.G

Mary says to me "what would you do if you won the lottery?" My answer..."keep sawmilling till its all gone"

Works for the Chinese.

I always teach the 'rule of thumb' as one inch per year + one year, the extra year being for the free water filling the centre of the cells rather than the moisture in the cell walls. The inch measurement is also to the centre of the timber as any section dries out from the top and the bottom at the same time.

Originally Posted by CAG

I always teach the 'rule of thumb' as one inch per year + one year, the extra year being for the free water filling the centre of the cells rather than the moisture in the cell walls. The inch measurement is also to the centre of the timber as any section dries out from the top and the bottom at the same time.

My issue with the rule of thumb is all the times it doesn't work, as opposed to all the times it does. It works well on 25 and 38 mm thicknesses that have been stored in average conditions. It mostly works on 50mm, unless we're talking dense tight grained hardwoods.

Thicker then 2" and it's pretty inaccurate, and the thicker we get the more inaccurate it becomes.
Mostly 19mm hardwood is at EMC to core in around 4 months.
25mm softwood is often EMC at core in 6 months.
Timber not stored with adequate ventilation and it's a fail althougher.
etcetera etcetera etcetera.
So I regard it as a poor rule of thumb at best. There are too many times it's just plumb wrong with timber never being dry in thicker sections, or time wasted in thinner ones.

A decent moisture meter with a species corrections chart is the best rule of thumb of them all, I've found.

There are two types of water in wood.
Free water, is water between the cells. It usually runs out quite quickly - days to weeks - and is basicly all the wood moisture above around 20%. Loss of free water results in weight loss only.
Bound water, is water within the cells themselves. It's the slow drying part as it needs to diffuse through the cell walls. It is the loss of bound water from within the cells and subsequent cellular size reduction that results in timber shrinkage.