I’ve written several times in the past about the green turning technique and process, most explicitly in this post. My consensus in the past seems to be that I recognize the value of green turning techniques but I have not enjoyed the actual process of finishing green turned bowls. Despite this lack of pleasure, I have continued to use the process because the reality remains that the VAST majority of turning blanks sold are some degree of green. They are almost universally sealed as well but I have had the unfortunate experience of seeing those seals, most of which, if guaranteed at all, are only guaranteed for 30 days, fail and thus I have lost blanks for which I paid, sometimes, significant prices.
At some point last year, I think in the autumn or early winter, I acquired 7 green cherry blanks of different sizes, the two biggest being 8″ x 4″. I no longer remember why I did this. I do love to turn Cherry but I can buy it kiln dried, which I find to be an easier product to deal with than green turned and dried. They were sitting on a table in the shop, well coated in glue and they were obviously dry based on the considerable warping evident in all of them. I have completed the final turning process and I have learned some valuable lessons from the successes and, most importantly, from the failures.
Of the 7 only 3 were successful for varying reasons. If you are doing the math you know that represents a rather dismal 43% success rate and I am very disappointed in that. But, the lessons learned were valuable and they will help me in the future when I inevitably end up green turning again even though I am not a fan of the process, mostly I think because I don’t have enough experience, and I especially don’t have enough positive experiences.
Green Turned Cherry Finished (the two bowls lower left had fatal flaws but I photographed them anyway before recycling them)
Lesson number one: Leave PLENTY of material for final turning. Two of the blanks failed because they became too thin. In the previous post referenced above I quote the 10% rule which I acquired from Craft Supply USA, i.e. leave 10% of the original diameter, or 1″ for a 10″ blank for example. I would dispute this estimation. I lost one of the seven because I cut it too thin although I was well within the 10% rule. The reality is that some wood species move more than others when drying and unless you have experience green turning a certain species you might not know how much movement to expect. And, the wetter your blank is the more it will move when drying and you have essentially no way to know how wet the blank is until you start to turn it. From experience, I know that Bocote and Cedar move hardly at all, Black Cherry moves a good deal more, Holly moves even more, and Persimmon moves the most of the woods I have green turned. There is a way to have some idea about movement expectations with wood you haven’t tried before and that is to consult the Wood Database. The Wood Database will give radial, tangential, and volumetric shrinkage rates, and tangential to radial ratios, for hundreds of woods. Black Cherry for example has a radial shrinkage of 3.7%, tangential of 7.1%, and volumetric of 11.5% with a T/R Ratio of 1.9. Holly has a volumetric of 16.9%, while Persimmon has a shocking volumetric shrinkage of 19.1%. On the opposite end, Eastern Red Cedar has a volumetric shrinkage of 7.8%. If you have experience with one wood you can compare a new wood to a known and have some sense of how much it will move comparatively. The average volumetric shrinkage percentage is between 9% and 15%. So, true to my experience Holly and Persimmon move a lot more than most woods and this is a known phenomenon. For a detailed explanation of these concepts refer here. My point being that if I know a species will move more I probably want to leave more material, perhaps more than 10%, when I green turn it.