Drying burls more risky than other parts of a tree because of crazy cell structure and voids due to disease or insects damage.
Since no idea how long ago burls were cut from a tree or size. Also how they were cut hard to answer your question. If freshly cut would end seal and weigh each burl until no longer losing any weight.
Since do not know what part of the country you are from or annual relative humidity for the area and size hard to give a time frame. So would advise using a scale to weigh those burls weekly until see no more weight loss.
I like to turn green wood, then pack it in a brown paper bag (like you might get from a grocery store) in it's own shavings. You can either weigh it periodically to see when the weight stabilizes, or (if you have a moisture meter - a wonderful tool to own) be confident that it is ready to turn when it reaches 10 12% moisture content.
Recently turned a piece of VERY wet Maple. Wrapped it in a damp paper towel and put it in a plastic bag. It dried just fine. However, I guess that the sugar in the sap wasn't happy with what I was doing because it turned to MOLD! Fascinating splotches all over the outside.
No problem to get rid of and the bowl dried very nicely.
The growth of fungi depends on suitably mild temperatures, moisture, and air (oxygen). At one time according to Forrest Product Labatory, I lived in zone 5 for mold growth in wood. They have updated that information now looking at a chart in chapter 14, figure 14-1 called Climate index for decay hazard. They assigned my area 80 out of 100 for mold growth. So take a look at chapter 14 Bio-deterioration of Wood and figure 14-1 to see where you stand. Scroll down the page to chapter 14 and click on.
I recently finished a few cherry burls that I seasoned about 6 months ago. One thing you should do is inspect the bowl first. Termites love these cherry burls, so what I typically due is put a couple of blanks in a trash bag and freeze it for a few days to kill any insects within the burl. After those few days, take your blank out of the freezer and thaw, then rough turn. As far as drying goes, I have rough turned to about 3/4 of an inch thick and put them in a 50 gallon trash bag filled with shavings and my other roughed out bowl blanks. I haven't lost one burl yet due to checking. This technique will allow you to open the bag as often as you please and check on the progress without having to rewrap your bowl in a paper bag or etc.
CSmith your method of freezing for insects ruptures the cell walls and lets the wood dry at a more even rate. They used to do the same idea by boiling green wood to rupture the cell walls so the wood could be worked green. It's the uneven shrinkage of the cells as they lose water that cause the problems for us. I would guess that the only problem you may notice with your method is that some wood may want to fuzz more than normal. Great idea.
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