Acclimatising wood before project

[macleod92]

Hi,
I’m getting a bit more into woodworking in general (mostly building furniture) and having been doing a lot of research regarding timber.
I’m a bit confused as to the best thing to do to let wood acclimatise.
I know that you should put the wood in the climate that it’s going to live so that its moisture content can get down to an appropriate level. But I was wondering what a reasonable procedure to follow would be.

My workshop is in an unheated garden shed (in the UK) and therefore has a different relative humidity to the house where it will be living.
So if I buy some wood from a merchant, should I just bring it into the house for a couple of weeks before taking it out to the shed to work it?
What if the project is going to take some time and therefore the wood is going to have to stay in the shed for a while?

Is this one of those situations where you can get bogged down in the literature and the worst case, but most of the time if you buy from a quality merchant you'll be fine?

Also what if you have a project that's faster turn around? Do you always have to leave the wood to acclimatise for a while?


Thanks in advance or the help!

 

[custard]

I guess a lot of people wrestle with the questions you're asking.

Start by thinking about it this way. Timber will move for two main reasons. First because of seasonal or climactic moisture changes. Second because you've done something to it, like you've cut it or you've wet one side and not the other, then stresses and tensions within the wood that previously were in equilibrium are now no longer balanced.

A lot of the commentary on contemporary woodworking comes from the US, and they generally have a much bigger problem than us when it comes to seasonal changes. We have a maritime climate, so it's fairly damp all year round plus we don't really get huge swings in temperature. So keep this in its proper context, we can't ignore seasonal changes, but neither do we have to be obsessed with them.

We do however have to pay attention whenever we process our components. If you need to reduce the thickness of a board then plane equal amounts from each face. If you plan on deep ripping a 2" thick board into two 1" thick boards then try and work with quarter sawn stock, or at the very least do it in the knowledge that you'll probably get some cupping and you'll need to then rest the board in stick for a few days before planing it flat again. And taking about "in stick", make sure air is free to circulate all around your boards, most especially when you start to get out your cutting list. If you find you're being plagued with movement problems then work down to finished dimensions in two or three stages over several days or even weeks. The implication of this is clear, if you need finished 18mm components don't start with 18mm boards, start with 25mm to give yourself some wiggle room.

There's also the fact that some furniture components demand far more care than others. For example the frame components, in frame and panel construction, need to be dead flat or you're guaranteed to suffer from twisted doors that never fit or shut right. However, the panel, provided it's thin enough to flex, isn't quite so critical. Likewise, if you aspire to piston fit drawers, then you need to be really fussy about the drawer sides, working them down to final thickness over several weeks if necessary. The drawer bottom though, and even the drawer front, aren't quite as demanding.

You mention timber yard quality. Lots of softwood yards store their stock outdoors. And very few hardwood yards keep their non-waney edged boards stickered. But above and beyond this is the fact that very few yards, even otherwise good yards, will take much care when they dimension boards. If you order planed boards be aware that it's normal practise to plane one face flat and then flip the board over and whack it repeatedly through the thicknesser until it's down to final thickness. That's a recipe for cupping, but unfortunately it's never stopped yards from doing it! The hard truth is no timber yard will ever cherish and coddle boards the way a furniture maker will.

So get in the habit of buying over thick sawn boards, buy them well in advance to let them acclimatise in your shed for a couple of weeks, process them responsibly, in particular respecting the need to balance your planing. Avoid deep ripping if at all possible. Identify the movement critical components of your projects, and take even more care with these. Do all that and as long as your shed is dry even if it's not heated then you should be okay. If the day comes when you're making Carlton House Desks or Long Case Clocks then you may have to re-think things, but by then I guess you'll have outgrown your shed anyway!

Good luck!

 

[RogerS]

And don't think that just because the board is 25mm thick and you're after 10mm that you can rip it down in half and then plane and thickness to get two lengths out. You are on a hiding to nothing...DAMHIKT.

 

[Sgian Dubh]

Much depends on what you plan to build for what location. For example, if you're building outdoor furniture here in the UK you'd be best to use either air dried wood which is typically somewhere between 17% and 22% MC, or allow your kiln dried wood to rise to these sorts of MC levels by storing and stickering it up outside under cover for two or three weeks. Typically, exterior woodwork hovers somewhere near these MC levels (or even above) throughout the year.

For the manufacture of interior furniture you're obviously therefore best to use wood that's been dried to much lower levels, either through kilning or acclimatising air dried material. Imported US wood is kilned to a target MC of 7% ±2%. European wood kiln operators aim for about 10- 12% MC. In almost all cases wood that leaves a kiln is close (aka dead) stacked, and usually shrink wrapped to hamper moisture regain, even when the material is stored in various draughty warehouses as it's transported from the kiln to the final buyer. This close stacking and wrapping is generally very effective. On the whole therefore, if you buy kiln dried wood from a reputable timber merchant there's little or no need to worry about acclimatising the stock prior to using it. However, if you're a very slow builder working in a cold and damp workshop you might need to consider strategies for keeping the MC relatively low during the build, e.g., bag parts in plastic, bring parts into the house between operations and processes, etc.

The following discussion about appropriate wood MC levels in relation to building decisions may be useful:

The thoughtful woodworker must take into account the following elements:
• The moisture content of the wood at the time of purchase.
• The RH range of the workshop or location used for building the project.
• The RH range of the location of the finished item.
• An understanding of the typical tangential and radial expansion and contraction the selected timber species experiences over a range of moisture contents.

Allowing for a greater range of RH than the items’ intended final home — these are conditions the item might find itself in later in its life after installation in the building it was designed for. For example a piece of free standing furniture designed for a specific house may be transported around the world in a container on a ship’s deck. Secondly, at some time in its life the item might be stored in a cool, damp barn at a coastal location, or in a hot arid storage facility near Alice Springs in central Australia.

In the UK air dried timber with a moisture content of about 18 percent (intended for a final location in a house or office building as a piece of furniture) must go through final conditioning to bring the MC down. Timber between 9 percent and 14 percent MC is reasonably stable in typical UK workshops. UK workshop interior conditions are often closer to rarely heated outbuildings than they are to house or office interiors. This means they are not particularly well insulated and heated.

If the wood is not conditioned to 13 percent MC or below, and it’s cut into smaller parts whilst still 18 percent MC it will almost always rapidly warp by twisting, cupping and bowing often making it largely unusable. This usually happens either during or shortly after the initial processing largely due to rapid moisture loss from freshly exposed wood. This tendency is sometimes compounded by the release of stress brought about during the drying process, whether it was air dried, kiln dried, or a combination of both processes.

Final conditioning happens in more than one way. First, the wood can be kiln dried by putting it through a short final drying cycle to reduce the MC to about 9-10 percent MC.

The second way is to place the full sized planks in stickered piles in the workshop allowing the air to circulate through the wood ‘acclimatising’ it until it reaches approximately 10- 12 percent MC.

A third method requires setting or marking out the parts for the project on the planks purchased. The parts are cut out of the wood generously over-sized to allow for substantial movement in the final conditioning. This method exposes more surface area to the surrounding conditions and exposes the core of the wood. This causes the parts to acclimatise more quickly.

Once the wood has reached something close to 10 or 12 percent MC the furniture can be constructed in the knowledge the current moisture content of the wood is closely matched to the EMC wood will acclimatise to in an average British workshop. 10 percent MC is conveniently in the middle of the moisture content range the wood will experience in service in most British houses. Workshops vary of course, and those with insulation and climate control have conditions similar to those found in typical British houses and offices. In this case it makes sense to work somewhat drier wood, e.g., wood at about 8 percent to 10 percent MC. Some workshops are extremely poorly sheltered from the outside weather and tend to be substantially damper with erratic RH values. Such conditions cause it to be rather challenging to make furniture destined for interior use. The RH conditions of damp workshops means wood will only dry out to about 12 percent or 14 percent MC. This places the maker in the position of knowing the wooden parts will initially tend to lose moisture and shrink when put into a household interior, but this is not a significant problem if allowances are made for expected shrinkage, and future expansion, during the making. Slainte.

 

[Chris152]

I had to check 'slainte' on the net. Really helpful post, I can see I need to make changes to my draughty garage/ workshop if I'm to successfully make the tables I'm learning to make. Thanks, and slainte.

 

[macleod92]

Wow, some really great advice there. Thank you so much everyone.

I think I’m a bit clearer on things. Another couple of questions if that's alright.

If you order planed boards be aware that it's normal practise to plane one face flat and then flip the board over and whack it repeatedly through the thicknesser until it's down to final thickness. That's a recipe for cupping,

custard you said that it was standard practice for boards to be planed on one edge and then run it through a thicknesser. I don’t have a jointer so I run stock through my thicknesser on a jointer sled (like this one: https://www.youtube.com/watch?v=XrUcHUqTSyM ) to flatten one edge, then back through the thicknesser to get the other side parallel. Based on what you said, this wouldn’t be great. With my kit in mind, what would be the best way to do things properly?

And don't think that just because the board is 25mm thick and you're after 10mm that you can rip it down in half and then plane and thickness to get two lengths out. You are on a hiding to nothing...DAMHIKT.

RogerS So if you did want to get thinner stock out of thicker pieces, how would you go about doing this?

UK workshop interior conditions are often closer to rarely heated outbuildings than they are to house or office interiors. This means they are not particularly well insulated and heated.

Sgian So what sort of MC will wood that's left in a workshop like this get to? Will it be the less than 13% required?

Sorry if these questions are very obvious. Thanks again for everyones help.

 

[RobinBHM]

The most important thing is to cut out your parts into rough sawn sizes and let them acclimatise for a while.

Almost all timber contains some degree of tension that is released when cut.

 

[Sgian Dubh]

So what sort of MC will wood that's left in a workshop like this get to? Will it be the less than 13% required?

In the workshop conditions you've described, given time, the wood is likely to move to somewhere between about 13 - 15% MC. I wouldn't think of this as being a major problem - you build in the expectation that artefacts destined for habitable spaces are most likely to experience some initial shrinkage during the first moisture cycle after installing in a habitable building, all depending on what material you're using and how dry it is when you start off. Thereafter the wood will shrink and expand as normal over annual moisture cycles. My own home workshop, as distinct from the professional workshops where I work, is a draughty and leaky garage, and I sometimes make bits and pieces of furniture in there without future problems.

As Robin said above, you can help by cutting all parts for a project generously oversized at the beginning of the project and allowing them to acclimatise to your workshop conditions. In the shorter term after this initial sizing you'll probably be able to get on with some of the building almost immediately whilst letting other dimensionally critical parts settle into their new size and perhaps shape. Slainte.

 

[dzj]

The most important thing is to cut out your parts into rough sawn sizes and let them acclimatise for a while.

Almost all timber contains some degree of tension that is released when cut.

That's the only kind of acclimatising I also do. As for moisture and such, just be aware
of crossgrain/ longgrain expansion issues and follow the suggested steps the craft recomends and you'll be fine.

 

[custard]

If you order planed boards be aware that it's normal practise to plane one face flat and then flip the board over and whack it repeatedly through the thicknesser until it's down to final thickness. That's a recipe for cupping,

custard you said that it was standard practice for boards to be planed on one edge and then run it through a thicknesser. I don’t have a jointer so I run stock through my thicknesser on a jointer sled (like this one: https://www.youtube.com/watch?v=XrUcHUqTSyM ) to flatten one edge, then back through the thicknesser to get the other side parallel. Based on what you said, this wouldn’t be great. With my kit in mind, what would be the best way to do things properly?

There's no board edges in this, it's just about the opposing board faces.

A timber yard will flatten off one face and then reference the required thickness from that flattened face, in other words the first flattened face won't get touched again, all the subsequent thicknessing will then take place on the opposite face. Sometimes this works out fine, sometimes it doesn't. The way you'd do it in your workshop with your jointer sled is you'd flatten one face, then flatten the opposite face, but if there's any further thicknessing needed to achieve your final required thickness you'd remove equal amounts of material from both faces, flipping the board between each pass.

The reason this is important is that most boards, and in particular thicker boards, are unlikely to be uniformly dry throughout. To minimise warping you want the outer faces to have the same moisture content, and to achieve this you thickness equally from both faces. What was the centre line in the thickness of the original board should, roughly at least, remain the centre line in the finished, dimensioned board.

This, by the way, is where deep ripping often goes wrong. Let's say you have a 50mm thick board that has a 10% moisture content on the outer faces, but a 15% moisture content in the centre. You deep rip it in half on a bandsaw to produce two 25mm thick boards (less the kerf of course). The problem now is that each of your new boards has one 10% moisture face and one 15% moisture face. That means they are virtually guaranteed to come off the bandsaw cupping like crazy. Leave them to dry and they might recover some flatness, but there will be so much internal chaos within the timber as it adjusts to this new state of affairs that you'll probably still end up with two curly wurly banana boards!

Hope that explains things better.

 

[macleod92]

Wow, that's a lot of great advice! Thank you everyone, I really appreciate it.