Whadaya do when a rough-out increases in weight?

[Glen Blanchard]

I have some roughed out bowls/vessels that increased in weight by a gram or two here recently. My logic tells me that, regardless of the results of previous weighings, regardless of how long it has been drying and even it its weight had not previously appeared to stabilize, when such a thing occurs, there is no explanation for it other than that it has reached an ambient moisture content, and as such, is 'dry' and ready for its second turning. Does my logic fail me? All this of course assumes no operator error. IOW, could such a thing ever happen having not equilibrated its moisture content with the environment it is in?

[charlie knighton]

I suggest that on the days you weigh, that you weigh each item 3 or 5 times with 5 minutes in between each weight,,,,,,see if you are getting same results each day

[Tom Wilson66]

The explanation is a property of wood, which is that wood is hygroscopic. That means that when the humidity in the atmosphere goes up, the wood will absorb moisture out of the air, and if the humidity in the atmosphere goes down, the wood will give up moisture. So I would say your wood is in balance with the moisture in the atmosphere, and a recent rise in humidity has caused the wood to absorb moisture and thereby increase in weight.

[Fred Belknap]

Pretty normal for this time of year. Wood that is in the shop, especially a heated shop, will gain moisture when the outside humidity goes up in the spring with the return of warm air.

[Glen Blanchard]

Tom, Fred - I pretty much understand what you have stated.

Let me create a hypothetical example. Let's say that my shop had been at 6.0% RH for all of April, and then jumped to 7.0% on May 1. Let's also say that I had a roughed out bowl that was at 8.0% MC on May 1. That bowl would not have increased weight with the rise in May's RH, would it? My logic tells me that even though the RH of my shop increased, this roughed out bowl would have still lost some water (and thus weight) as its MC was still higher than ambient air. Furthermore, the only way that bowl would have taken on water (and thus increased its weight) is if it had already reached 6.0% (or something under 7.0%) the ambient RH - which would mean that it had already finished drying.

So I ask again, where is my logic flawed?

[robert baccus]

I think you will find that air humidity and wood moisture are on different scales. Wood is compared to oven dry weight and air to what moisture % it will hold at that temperature . Here in the south our summer RH runs in the 70 to 95 % range and shop wood is considered to be dry at 10-12%. The numbers are not comparable.

[Reed Gray]

Well, most dry wood, depending on where you are, will stabilize in the 10% to 15% moisture content. Way different in Palm Springs than here in Oregon. You can usually only get it lower than that if you are in a really dry place of have a really dry shop. It will also be in that range fresh out of the kiln. Most lumber, when air drying, requires 1 year per inch of thickness, and then another 6 or so months in the shop, and it will be stabilized to ambient conditions. So, if your bowl blank has been sitting around for more than a year, it is most likely as dry as it will get. A weight change of a gram or two for a piece that weighs a pound or so (450 or so grams per pound) is not significant.

robo hippy

[Dale Miner]

I once had a 14" cherry bowl roughout that had been drying in my basement for a couple years. Took it to the non climate controlled shop, second turned it, sanded it, and put a few pencil marks on it with the dividing feature of the lathe with plans to embelish it later. I left it in the chuck and it was a couple of days before getting back to it. Those days were rainy with a 100% relative humidity. When getting back to the bowl, the bowl was 9/16" out of round, with the smaller dimension in the end grain direction. The bowl had absorbed enough moisture to actually gain that much dimension. Once back in the house for a few weeks, it returned to round.

[Bob Hamilton]

If I have been tracking the weight of a rough out and the weight starts to go back up after trending down then I go ahead and finish turn it. As I see it, that is a sure indication that it has reached equilibrium moisture content with the ambient conditions.

Take care
Bob