Minus 7c and seasoned timber?

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devonwoody

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I've got around 5cu.ft of hardwood in my workshop/garage and its moisture content was down to around 11 per cent at the end of last summer,

What would minus 7c have done to the timber with that amount of mositure?

Outside I had a log that has split, bay tree log around 5ft in length and green.
 
Probably have pushed it up a bit I would think, but nothing physically I wouldn't imagine.

There is a thread on relative Humidity from a few days ago which might help. Do you have a moisture meter to test it with?
 
Remember timber is like a sponge...it'll soak up moisture if the weather is wet and give up the same when the weather's dry. So if your timber was at 11% it will now be a higher MC (as it's winter and wet) than the summer - Rob
 
woodbloke":1w503ltf said:
Remember timber is like a sponge...it'll soak up moisture if the weather is wet and give up the same when the weather's dry. So if your timber was at 11% it will now be a higher MC (as it's winter and wet) than the summer - Rob

but what about the frost effect?


timber merchant used his moisture gauge when I purchased.
 
To answer Devonwoody's question, don't worry about the frost. It won't have done any harm and would nt affect the moisture content.

Jim
 
I saw Pine trees in Canada that had exploded because of frost. You would think someone had detonated explosives against them. It mostly happens to young trees and has to do with too much sap and severe frost before the sap decreases for winter.

Brendan
 
Trees do occasionally explode in colder climes I'm told, but they need to get down to -40 or so.
I bought some 4x1 sawn from a local timber yard when it was really cold, and the timber was definatly frozen - some of the sawdust melted when I'd cut it. It didn't effect the wood in any serious way, and now it's thawed out looks completly normal.
 
Back in the 19th century when the Yanks thought they were on a mission to fell and convert their 'endless' forests as quick as they could, they would file the saws differently in winter, so they could cut frozen wood.
 
woodbloke":3llijuhg said:
Remember timber is like a sponge...it'll soak up moisture if the weather is wet and give up the same when the weather's dry. So if your timber was at 11% it will now be a higher MC (as it's winter and wet) than the summer - Rob


Robs said all you need to know,
there shouldn't be enough moisture in converted timber for frost to effect it.

JHB
 
coincidentally I was watching Canadian TV programme last night that showed the production of plywood. The first step for the logs after entering the factory was a hot wash, to clean off the mud and to thaw them out!

Roy.
 
The effect of freezing on the drying/dryness of wood is a bit complicated. There's a few hiccoughs that make it hard too call exact effects from the theory, at least at my level of knowledge.

What happens is roughly as follows:

If your shop was full of air at 80% RH and at say 55 deg F (typical mildly heated winter conditions) then dropping it to below 40 deg F would result in saturation or 100% RH. i.e the ability of the air to hold moisture becomes less than the amount of moisture present (the dew point is reached), and the resulting dew/fog/free moisture would quite quickly wet the wood. If this condition was maintained the EMC (equilibrium moisture content) of the wood would over time reach the fibre saturation point which is around 30%.

Keep on chilling the air down and you get more and more of the moisture in the air coming out as dew, but the same broad situation pertains.

Once you get below freezing/the triple point/the frost point the dew starts to drop out directly as frost. Once below this water can no longer be present in liquid form, and your wood cannot take up any more moisture.

I don't know the psychrometrics exactly (it's about maintaining the right partial pressures), but if say you were to bring that piece of frozen wood indoors into a warm room at low RH (the likely result of heating frozen air taken in from outside that does not by definition contain a lot of moisture) you would likely get sublimation or freeze drying of the wood - the frozen water in it would convert invisibly to water vapour, mix with the air passing through the room and be taken away. i.e. your wood would dry out, but how quickly i don't know. The rate of drying would fairly quickly slow down (once the wood had warmed up, the ice melted, and its sublimation consequently stopped),but the wood would as you would expect eventually air dry as normal down to a very low moisture content.

So it's a bit of a swings and roundabouts deal depending on not just the exact conditions, but also on how long the wood is exposed to them.

On the face of it though periods spent at temperatures just above freezing/around the dew point are likely to result in what could be quite large increases in moisture content - but i don't know how fast wood takes up moisture in saturated air. It'll be the worst case (fastest possible) situation for moisture gain though.

Once frozen the situation is pretty much stable (no moisture can be taken up as its locked into ice), but held for long enough in the right conditions then warming (while staying below the frost point) should result in the loss of quite a lot of moisture by sublimation. (the frost converts directly to water vapour in the air, but without becoming liquid on the way through)

If it stalled slightly above freezing again on the way up, then you would probably (depends on the room conditions, but presuming the same air/moisture content you started with) be back into saturated (dew filled) air, and back to the rapid take up of moisture.

On the other hand a quick zap from normal conditions down to freezing, followed by a rapid return to normal conditions when the freeze ends would likely have very little nett effect.

I do know there has been work done on the freeze drying of wood in naturally occurring Winter/Spring conditions in Northern Canada.

As long as the wood is not 'green' (it's below the fibre saturation point or ~30% MC, so that the cells contain no liquid water, and all moisture is contained in the cell walls) freezing apparently does it no harm....

ian
 
Thanks Ian, you obviously have studied and understand this process.

Still, I don't think I will bother popping out to the shed to try some of those dovetails Derek C posted earlier this week.
 
Helps DW if its warm enough for your fingers to work!! :!: I was trained years ago as a HVAC service engineer.

Whatever the plan I'd say stick a meter in it too if it's been around in very cold but not freezing conditions in a shop for a while - it's probably worse than being out doors in cold weather in that it likely traps air (from the preceding warmer weather) with quite a lot of moisture in it, and chills it down so you get a very high % RH which is a fair measure of the rate and amount of moisture the timber will take up. (see PS below)

These are the worst case conditions for corrosion of machinery too. When this happens its not unusual to go into a shop and find water beaded all over metal surfaces - which then rust like crazy.

%RH tends to be lowish/very low outside in frosty weather, which is why in some ways the wood would be better outdoors. (tough if it rains unexpectedly though)

Which explains why keeping your shop moderately ventilated (to avoid trapping humid air), and with some heat on is the best way to avoid corrosion problems.

Raising the temperature of even very humid/wet air from just above freezing to say even 45 deg F (which is still pretty chilly) will drop the %RH from close to 100% to around 20% which is actually considerably dryer than we experience in normal weather conditions at any time of the year in this part of the world...

ian

PS To explain %RH and how it influences EMC for anybody that's interested. (i've been studying R. Bruce Hoadley's 'Understanding Wood' lately which many of you are familiar with - it dovetails nicely with my limited HVAC background)

The actual (or 'absolute') amount of water in the air can be measured in kg water/kg of dry air or similar units. (e.g. lbs water/lb of dry air) The is the absolute or specific humidity. Various volume based measures get used too.

But as the temperature rises air can hold much more water, and as it cools much less. As you cool air it continues for a while to be able to contain the water as vapour (invisible and gas like, with no tendency to wet surfaces), but it moves closer and closer to what is called saturation or the dew point. Once the dew point is reached the surplus moisture forms water droplets (dew/fog) which drop out on to handy surfaces like your machines, and your wood.

The measure of relative humidity or %RH is used to say how much water vapour the air has in it versus the max it can hold at 100% RH. (for that temperature) When the dew point or 100% RH is reached the %RH can increase no more, and as above the water separates out as droplets. 0% RH means the air is completely dry and contains no moisture at all.

%RH is the commonly used practical measure of humidity, because it's in the end the best indicator of practical effects - of wetting effect, of how hot and bothered or cold we feel, of risk of mould, of corrosion or whatever. Most especially of the %EMC or equilibrium moisture content of wood - what % moisture the wood will take up (%lbs water/lb of oven dry wood) if left to take up as much moisture it can for a given % RH.

Luckily the relationship between %RH and EMC varies very little between different types of wood - 0%RH = 0% EMC; 25%RH = 5%EMC; 50%RH = 9%EMC, 75%RH = 14%EMC and 100%RH = 28 - 30%EMC. (within a few % EMC that is)

28 - 30%EMC happens to be the fibre saturation point, or the maximum amount of water the wood can take up from the air. This is all absorbed into it's cell walls, and called 'bound' water. This and any moisture content above is referred to as 'green', as it increasing the water beyond the FSP makes no big difference to its working characteristics.

Freshly cut timber has higher moisture contents which vary with the season and species - it contains 'free' water in its cells as well as in the fibres or cell walls - the sap.
 
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