I have been weighing several green turned bowls about once a week for the last five or six weeks to determine if they are dry enough to put finish on them. Their weight was going down regularly. I weighed them yesterday and they are all nearly back to their starting weight. Several had regained multiple ounces. They are setting out on a workbench in my basement shop which is air conditioned. I checked the digital scale with some known weights and it appears to be correct. Has anyone else experienced this? We have had a lot of rain this week. When is it okay to finish them?
Put a hygrometer in your shop. Just because it is AC-ed, doesn't mean the air is bone dry. It's a process variable that needs to be understood if you're air drying.
The wood is drying to the ambient air moisture content, and will fluctuate now with the local humidity. Assuming it has lost a lot of its moisture till now. If that's the case, and the charted moisture loss has gone from a near vertical line, to a near horizontal line, tis time for finish.
Wood gains and lose MC with changes in relative humidity in the area. Even after finishing these changes occur. Finishes only slow down this water vapor transfer but don't stop it!
. . "they are all nearly back to their starting weight. Several had regained multiple ounces."
Either someone is putting this wood in water under pressure over night or aliens have invaded the shop, choosing to rehydrate this wood with their hydro-ray guns. Wood does not loose free or bound water over weeks and then regain it overnight from atmospheric conditions.
I probably hadn't weighed them for six or eight days and we have had almost constant rain. But I posted because of the extreme, at least I thought, change. I did not expect it. I usually just let turnings sit around for a while and finish them when they feel dry. This time I thought that I would be more scientific and weigh them. I really thought something was wrong with the scale. If it ever stops raining, I'll finish them when the weight goes back down.
Get yourself a moisture meter (pin type is fine and cheap at Lowes, etc. When the bolw's MC matches the wood in your inside door jamb, you're good the finish. Rain outside's got nothing to do with it unless your roof's leaking and you're using the bowls to catch the drips
Here in central NC, my air conditioned basement shop averages a temperature of about 72 -74 F. In the summer, the humidity can reach 70% measured on a digital ...whatever the thing is called. And of course, the humidity is apt to be higher when it is damp and rainy outside.
I too have observed that bowl blanks will gain weight during a high-humidity period. However, I have not paid enough attention to the phenomenon to actually calculate a percentage or even record the weight gain. I just figure the blank has reached equilibrium and is readjusting to the increased humidity.
When I observe an increase in weight, I conclude that the blank is ready to finish turn.
In winter, the humidity can drop to as low as 30%, due in part to a wood stove I use for heat. Usually, but not always, I try to keep the humidity close to 50% by running a dehumidifier in the summer and a humidifier in the winter. I figure either extreme is less than ideal.
~Doc Edit: The numbers that Walta gave above are typical of what I observe as a bowl blank dries. His last three values are 934, 919, and 908 grams. When I observe a blank to be gaining weight, the reading might increase from 908 back up to 919 or even 934, using Walta's numbers as an example.
Don’t know where you live or relative humidity for your area but wood will gain and lose MC simply based upon RH. So could see where storing blanks in a basement even with air conditioning might affect weight of your blanks.
Depending upon species some of my roughed bowl blanks ready to go in 3 to six months in my unheated-air condition shop. May not get around to them for over a year. Only seal blanks when processing them once roughed out just set them aside to air dry.
If had a ball bark MC meter reading before rough turning and now might explain what you are seeing better. Weighing also a great method to determine when wood at EMC.
Back when this meter was the darling of woodworking message boards picked one up at Lowes for $10. Works good and would still recommend it for ball park numbers. Think gets a 4-star review rom users at Lowes.
General Tools & Instruments Digital Multimeter
Item #: 78059 | Model #: MMD4E, $29.98
Doubt would work for freshly cut down tree when sap running. Most consumer grade MM either don’t work or not very accurate then.
Many years ago, some experienced members of our yet-to-be-formed woodturning club told me that our average relative humidity here on the central east coast of Florida was +/- 13%. I had cut and salvaged many blanks from a summer when we had two hurricanes and thousands of trees were on the ground. I rough-turned about 40 blanks, mainly Seagrape, and double-bagged them with chips and shavings and wrote the records on the paper bags themselves. I was happy to watch them all lose moisture quite quickly until they reached EMC and them they would vary considerably on a daily basis. Then I would quit worrying about them and make more blanks. That is an old tried and true method and I found that the type of wood didn't really matter, whether seagrape, oak, or some tropical wood that was salvaged, they all followed the same track to EMC. In all that time I think I may have lost one blank to cracking. Amazing! I wonder how your wood would fare if you took it out of the air conditioning and left it where the wood could breathe normally?!
If look at figure 14-1. Climate index for decay hazard page 2 of Wood Handbook.
Barry I avoid plastic bag storage due to where I live and mold & staining and average relative humidity. The average relative humidity runs about 85% AM and 57 PM. Looking at figure 14-1 would think your risk of developing mold and stain would be higher.
If plastic bags work for you please stay with it, I only use them with wet wood is on the lathe and breaking for short period. Wood loses MC through evaporation caused by air circulation. So fans & air conditioner can only help moving air thus aid drying wood.
If lived in 29 Palms CA would have no problem with plastic bags.
