Bowl cracking

[Dave Jain]

I bought a 6x6x3 Indian rosewood blank from one of the large online lumber companies. Turned most of it,went back next night and it had cracks in it. I had it covered with shavings in a paper bag. It was completely sealed with wax when I got it. Years ago I bought blanks and never got cracks. What is the best way to avoid these cracks? Being sealed they will never dry, most blanks I've seen are end sealed only.


Thanks

[Chris A Lawrence]

If you cant finish it in one turning session put a plastic bag over it and loosely tie it around the chuck. That will keep it from drying out for a day. I have not gone more then 2 days doing this any longer i would wax the entire thing.

[John K Jordan]

I bought a 6x6x3 Indian rosewood blank from one of the large online lumber companies. Turned most of it,went back next night and it had cracks in it. I had it covered with shavings in a paper bag. It was completely sealed with wax when I got it. Years ago I bought blanks and never got cracks. What is the best way to avoid these cracks? Being sealed they will never dry, most blanks I've seen are end sealed only.

Thanks

Bummer! Did you intend on continuing turning on that piece the next day? If so and if the wood is not dry, it needs to be kept wet between turning sessions. More about that below. You didn't say but I'm assuming you were turning a bowl or some type of hollowed form. Following are some general notes based on what I've learned and experienced.

Almost any wood will crack when drying due to the stresses from unequal shrinkage. Several factors are in play including the T/R ratio, the size of the blank (in particular the size of the exposed end grain), the strength of the wood, the pore structure, the actual moisture content, and the environment (temperature and humidity). If interested in learning more about this, the best book I've found is "Understand Wood" by R. Bruce Hoadley.

Always assume blanks covered with wax are sopping wet until you know differently. Note that when you buy blanks completely covered with wax they can be anywhere from freshly green to very dry. One dealer I know waxed some pieces because they sold better that way. He didn't know the moisture content but I did an oven-dry moisture measurement and found the blanks to be air dry for the season in our part of the country.

Dealers of exotic woods often seal on all sides. Indian Rosewood is usually moderately stable but some species are horrible. An exporter from Peru told me that some of the blanks I bought from him (not sealed but wrapped in plastic) were so unstable they HAD to be boiled to keep them from cracking.

Note that even covered completely with a heavy layer of wax blanks will still dry, but slowly. The slower you can dry the blanks, the better since when the moisture escapes slowly near the surface the moisture deep inside has more time to migrate outward keeping the entire blank about the same moisture. The cause of cracks is uneven shrinkage, far worse at the surface of the end grain in most blanks but also can happen where transverse grain is parallel to a flat side, where heartwood and sapwood meet (in some species), and where figure is wild. To cover all bases, the dealers have found it safer just to wax the entire blank. Well behaved woods such as straight grained walnut, maple, and other domestic woods (and many exotics) are fine with just the end grain waxed. I always wax the end grain, heavily wax some end grain, and wax other surfaces that experience has taught me might need it.

I track the weight of many dozens of blanks this way, both blanks I've cut myself and wood I've bought. Note that if I have several of the same species nearly identical in size, I track the weight of just one or two since they will all dry at nearly the same rate. Also note that it's important to allow air space all the way around the blank - I put them on wire shelves or loosely stack or sticker as space allows.

422567

There are several ways to avoid these cracks in turned work, whether turning spindles or bowls. I don't know which is "best" for you.

My favorite is to just let the blank air-dry to EMC (equilibrium moisture content) before using it. This does take patience since some large blanks might take years to dry. The patience is easier since I have many shelves of blanks in various stages of drying - the dogwood I'm using now has been on the shelves since 2006. Note that drying is not too useful for huge wet blanks which should be turned as soon as possible. Most of what I dry is for spindles and relatively small pieces - many are spindle blanks, larger pieces 2" thick for platters, 2-4" thick for most bowls, and a bunch of 3x3 and larger for lidded boxes, small forms, and such. However I have some larger, which are drying successfully, some up to 10x10x15. Note that spindle blanks for many domestic species can reach EMC in just a few months.

It is easy to determine if a blank is dry, the only thing needed is a scale. A good moisture meter is also helpful but the method with a scale can be more accurate. I like to use a digital kitchen scale that will measure to 22 lbs so I can track larger blanks. I put a piece of masking tape on the side of the blank and weigh it, recording the weight in grams and the date. Some months later I weigh it again and also record the weight and date. The weight will be less, even if 100% covered in heavy paraffin. Then I wait a few months and weigh again. Repeat until the weight doesn't change and the blank is dry, well, as dry as it will get in that environment. Note that sometimes the weight actually goes up! This indicates the wood is dry but the environment has changed, typically more damp in my shop in seasons when I'm not running AC.

BTW, when I determine a blank is partially dried (based on the weight) I often scrape off some or most of the wax on the side grain only to let it dry a bit quicker.

Another way to avoid cracks in bowls is to rough turn the wet wood to somewhat thicker than needed and try to dry slowly before finish turning. Some strategies are: seal with wax inside and out, wax or apply plastic wrap on the outside but not the inside, seal just the end grain, place in a plastic bag and invert the bag each day so the inside is outside.

A very successful method of preventing cracks in a roughed out bowl is to boil it, one guideline is 1 hour of boiling per inch of thickness.

If you start a piece from wet wood and want to take a break for a few hours or overnight, mist it with water and wrap tightly in a plastic bag. This will prevent rapid drying which will cause cracks.

I'm not much of a fan of shavings and paper bags, although many use those methods successfully. I do put put waxed roughed bowls in paper bags sometimes for drying, sometimes just throw in a box or tub or put on a shelf. I wax with Anchorseal. For a heavier coat of wax I let some of the water in the Anchorseal evaporate which thickens it.

JKJ

[Richard Coers]

In a paper bag with shavings is a method for drying, not retaining moisture. As Chris said, plastic bag. If I was doing rosewood, I would even spritz it with water before putting it in a plastic bag. You don't have an address listed, but if your furnace is running, even worse conditions for rosewood!