What causes wood to warp?
I think before any attempt can be made to straighten wood that has warped, you need to understand what caused it in the first place. So before we go into some different methods of non-intrusive straightening, ask yourself this question: what is wood? Wood is a material made of cells and fibres that swell and shrink depending on the temperature and moisture in the surrounding air. This takes place to different extents in different directions. All solid wood expands & contracts, but to different degrees with different species.
When a tree is first cut, the water in between the cells evaporates, then shrinkage sets in as some of the water within cells dries out. Drying from its green condition, wood will retain its original dimensions until the moisture content falls to about 30%, from which point it will shrink as it dries. A certain percentage of water always remains. With thorough air-seasoning, the moisture content of most types of wood will fall to between 25 and 15 percent, at which level it will be in balance with the moisture content of the air around it, and would be stable enough to be made into furniture.
Have you ever taken notice that antique oak furniture has not suffered as much from warping as other wood species, while mahogany furniture for example seems to have more than its fair share of such problems. The principal reason for this is that much of the wood used for oak furniture was quatersawn, so that the width of the board followed the radius of the log along the medullary rays, at right angles to the annual rings. This cut produces a board which is still liable to shrink but not distort.
When a panel was made from quartersawn cut wood, it would be fitted in a frame without glue so that with seasonal environmental changes it could move freely. If such a panel has split, it is probably because movement has been inhibited in some way.
In the case of mahogany furniture, there was a demand for the widest board possible, which often meant taking a piece from the full thickness of the log. Such a board was cut through the growth rings of a tree at varying angles across the width, therefore exposing it to problems of distortion.
A cool environment with a relative humidity of about 55% and a little bit of air movement is said to be ideal for wood and antique objects in general. A sudden change in the circumstances is the worst thing possible as the moisture will be sucked out of or pushed into the material far too quickly causing even more tension inside it.
Now that we have an idea why and how wood warps, what can we do about it?
Step one is to look at the role of an object and decide whether any action is necessary. If so, step two is to investigate the possible ways of gaining access to the warped component while at the same time considering the consequences of any other components involved. Then, having decided on the kind of treatment, we must think of a way to stabilize the object in its new shape and how to re-assemble whatever had to be taken apart.
Non-intrusive treatment would be changing the environmental circumstances, with little risk of damaging the object further except perhaps for transport. When a piece of furniture with a lot of shrunken components is placed in a damp environment, it might just return to its original shape.
In the case of a warped door which does not close properly anymore, shifting the hinges is a possibility. Another non invasive fix on a door, and one that I have seen work often, is adding a single or multiple catches to the warped end of the door. There are numerous options, such as the three pictured below.
In the case of a warped drawer, we could consider doing something about the drawer runners
Depending on the reason why the wood has twisted, another option to consider is that we can extract more moisture on the convex side or add moisture on the concave side. Humidity and heat partially soften the cells in the wood, allowing the wood fibres to slide along each other. After cooling and drying, they return to their initial state and the wood maintains its new shape. Forcing the wood into a slight counter twist in the cooling process is an aid but not a guarantee to prevent twisting back. This is a non invasive fix I that will work at times, but be advised that by no means is it 100% The problem is that the wood must be unfinished to allow the moisture to penetrate into the fibers. Even wood that has been stripped of its finish will have blocked pores limiting the effect of the moisture penetration.
Extracting moisture out of a convex surface could be undertaken with a household iron. It takes precise judgment to warm up the wood evenly with this method. Other options may involve an electric blanket. Wrapped in such a blanket we can leave the wood clamped in a certain position.
When the panel to be treated is a thick one, sometimes the use of steam is required, for which purpose we can use a steamer.
A very common problem is the warped tabletop, which has been finished on one side. Moisture does penetrate shellac and many of the other classic finishes, but at a highly reduced speed. So the underside will have moved more than the top.
If concave cupping is extreme and the integrity of the structure is compromised, a treatment of repeated wetting and drying the convex surface may be applied. Generally speaking all these solutions are only temporarily, as the wood will want to return to its disfigured state sooner or later. This phenomenon is called spring-back. The chances of experiencing a spring-back reaction are reduced when we countertwist the wood and leave it countertwisted for a long period of time, say a week or two. Because not only the cells on the concave side are compressed, but those on the other side as well, the tension in the wood is spread more evenly and chances of a return of the problem are minimized.
Intrusive methods are the next option, but require more skill and a firm understanding of wood and the consequences of our actions in relationship to wood movement, and restricting such movement.