Q: We are having a problem with shrinkage. We make furniture, but someone else sells and delivers it. This person claims he did everything correctly, including opening the furniture wrapping (we wrapped the furniture with shrink-wrap and it was fairly well sealed) and letting it acclimate to the house climate. When the customer moved in, they said the furniture looked really wonderful, but within a week, it started to warp, open joints and crack in a few places. We are so careful to keep our plant at 40 percent RH and check the MC of the lumber. This is frustrating! Can you help?
A: This seems to be an age-old problem for some people. Let's review the basics. First, wood does not shrink or swell unless its moisture content changes. From the information in your letter, you apparently already are aware of this. Your plant is just a little bit more humid than I would like; you are running at about 7.5 percent EMC. Can you lower it to 7.0 percent EMC or 35 percent RH?
Or maybe even 30 percent RH and 6.0 percent EMC? In the wintertime, this will be just a bit more reasonable. Your customer's home often is going to be 6.0 percent EMC (30 percent RH) in the wintertime, so running your plant a bit drier will make less of a change between your plant and the customer's home.
This plant RH change will not be very effective, however, if the main problem is that the incoming lumber is higher than 7.0 percent MC, on the average with 95 percent of the pieces between 6.0 percent MC and 8.0 percent MC. How closely do you check the incoming lumber's MC? Remember, in a kiln there are perhaps 15,000 pieces of lumber and the operator has only checked 10 or 12 of them. It is indeed possible that your lumber is too wet at times for a very dry home.
I do like the fact that you are wrapping the furniture, so that it cannot change MC in transit. A poly bag or shrink-wrap will prevent any significant MC changes, even though the outside conditions vary in temperature and RH.
I do wonder about what the sales person is doing however. If he is unwrapping the pieces before the people move in and the heat is not turned on (and maybe the paint, drying wall taping, and concrete still are drying), the furniture actually will be in a high RH situation and will pick up moisture.
Then, when the home owners move in and the heat is turned on, the sudden, large drop in RH and MC will cause a lot of shrinkage, especially compared to a much slower MC change.
Try to avoid large, fast MC changes if at all possible.
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