wood drying indoors: humidity & temp

[Big Beech]

managed to get going with the spalted holly and sort of got carried away and made me a small table. http://arbtalk.co.uk/forum/woodcraft-forum/56194-spalted-holly-worm.html

Outside temp has been around 6deg during the day with humidity of 56%.

So ive brought the new addition indoors and placed it in our cool hall way which is 7.5deg during the day and a humidity of 68%.

The item has been sanded and had BLO added and ive covered the ends flush with paper bagz.

So, the question is is there two wider gap in the humidty which will cause excessive cracking? And which is the main factor. Heat or humidity

Thx simon

Edited April 1, 2013 by Big Beech

[openspaceman]

So, the question is is there two wider gap in the humidty which will cause excessive cracking? And which is the main factor. Heat or humidity

 

Someone with more practical experience will probably be more helpful but IMO the two are intrinsically and inextricably linked.

 

Cracking is one of the simpler drying defects to explain and it is caused by two different phenomena. The first one is that the outside surface of the wood dries to below its fibre saturation point before the middle does. Now wood doesn't shrink much while all the water in the cells and vessels dries out but once that has gone (at about 30%mc) the water that leaves is associated with the cell walls and this causes the wood to shrink. So if the middle is still wet and full size the surface cannot shrink around it and so a crack is pulled. Temperature affects how fast the water migrates through the wood and also increases the water holding capacity of the air around the wood. The art is to balance the RH and temperature such that the moisture is moving from the middle to the outside at the same rate it is moving from the surface to the air.

 

The next thing is that wood shrinks more tangentially than radially and not much at all in length. So even if you are careful about drying below the fibre saturation point a baulk which boxes the centre of the log will still have tension in the faces which will lead to cracks opening and closing as the humidity changes. This is one reason for quarter sawing the tangential shrinkage is then across the thinnest section.

[howard]

Post by openspaceman was helpful and confirmed my fears regarding some large douglas that I have started milling and stacking inside the barn housing the mill. It is cracking and is clearly loosing too much water too quickly and the core can't catch up. As I am unable control RH in the barn (V hot at the moment) and I guess my only option in to try to store outside under shade.

 

I am just commissioning the mill I have designed and built from scratch over the last 4 years in my spare time. Cutting 5m long 600mm dia douglas bars trying to produce big beams and columns.

 

Any further advice would be appreciated.

 

Howard

Edited July 13, 2013 by howard
spelling

[farmer rod]

if it is already stacked inside you could cover with dpm, this would stop air movement which will reduce drying rate, obviously if its too well covered you will get mould. Nothing is simple!

[openspaceman]

if it is already stacked inside you could cover with dpm, this would stop air movement which will reduce drying rate, obviously if its too well covered you will get mould. Nothing is simple!

 

Rod beat me to it, it's about controlling the relative humidity, if it's hot then you need to maintain a higher RH under the damp proof membrane but you will still need controlled airflow. It's not the heat that causes the cracking it's the amount of moisture getting from the log to the air.

 

Also the cut of the timber will make a big difference, boxed heart will always have more problems that something sawn down the pith as it cannot releave itself by cupping.

[Big J]

You basically just can't cut high quality timber in this heat. It will crack and distort far far more than in winter. Wait until it cools down and do other jobs until then. It's frustrating, but not as frustrating as wasting good timber.