Jake":2uxnypnc said:
To be fair, I wasn't really trying to tell the whole story. You place a lot of stress above on the effect of heat (alone) on the lignin, which is all right and proper, but it could be read to imply that steaming a low moisture content plank is as effective as steaming a high-moisture content plank - which isn't the case in my experience..
It’s well known that steam bending is easier to do with green wood, i.e., wood at FSP. The next most compliant wood for steam bending is air dried material (between 18%-25% MC) and, least easily steam bent, is wood that has been dried to 6% or 7% MC; this is because wood kilned to 7% MC, the north American standard has become much stiffer and, due to a form of hysteresis, it is quite difficult to make wood soft and pliable again once it has reached this level of dryness and stiffness. Green wood at the time of introducing steam to it is already relatively soft and flexible compared to very dry kilned wood. Probably the best choice for steam bending is air dried wood at about 18-23%MC. This is because if very wet wood is bent, i.e., wood well above FSP with significant quantities of liquid in the cell cavities, the bending can cause bursting of the cell walls as any fully charged cells compress and disrupt. Such disruption leads to unnecessary loss of strength in the wood, which is undesirable. There’s less cell wall damage caused during the bending process if there’s ‘wiggle’ room inside the cell itself due to it being empty—the condition that air dried wood below FSP is in.
Not only that, but compared to green wood, air dried wood has had chance to reveal some of the distortion, checking, splitting and other faults that can occur during the drying process. Selecting air dried parts for bending that are in good shape at that stage means they are likely to go through the heating, steaming and wetting process and subsequent re-drying and cooling after bending with less risk of such faults showing up in the finished bend.
The kiln-drying bit is interesting - what do they do exactly to relieve case hardening? Inject steam?
Case hardening relief takes out any case hardening stresses that have built up during drying. During early stages of the drying process the shell of the wood tries to shrink as it loses moisture. It’s prevented from doing so by the core of the timber which is still wet and full size. By the end of the drying process the wood should be equally dry throughout and the core will have shrunk. The final conditioning adds water to the shell very quickly usually through hot steam injected into the kiln. The shell attempts to swell in response to the added water, but now, because the core has shrunk this prevents swelling of the shell. This attempted later swelling of the shell mirrors the early stage efforts to shrink tighter on the core. The net effect is that the initial shrinkage and later swelling cancel each other out and the casehardening is relieved.
Throughout the casehardening relief process kiln operators check progress by taking samples out of the kiln and doing the fork or prong test, see section 9.1.2 for further discussion and illustrations. When this test indicates the wood is stress free the job is complete. If the job is not done correctly two faults are possible. Firstly the casehardening procedure may be misjudged leaving the wood still casehardened. Secondly, it’s possible to overcompensate through adding too much steam and heat leaving the wood reverse case hardened.
Higher temperatures encourage casehardening relief through softening the lignin in the wood, the ‘glue’ that holds the cellular structure together; this allows the cells to slip and rearrange themselves into a stress free condition. As the wood later cools the lignin sets again holding the rearranged cells in their new position. This high heat treatment plus steam has parallels with steam bending wood around a former to create a permanent bend. It’s generally only possible to repair casehardening in conventional kilns capable of creating the necessary heat and humidity. Low temperature dehumidification kilns and solar kilns are two types that can’t create the right conditions; to set against this limitation these latter two types of kiln are less likely to induce casehardening in the first place because they operate more gently and at lower temperatures.
Slainte.
