Green wood, wet shavings and a bowl that cracked overnight
The first bowl I turned from green wood was the most pleasant hour I have spent at the lathe, and the next morning it was in two pieces. Here is what went wrong, so that it does not happen again.
The wood was a section of maple, fresh from a tree felled that week, about 20 cm across. Turning green wood is nothing like turning dry wood. The gouge goes through it like a knife through firm cheese, the shavings come off in long wet ribbons that wrap around your forearm, and the whole garage smells sweet and sour. I had the outside shaped and the inside hollowed to a finished wall of about 6 mm in under an hour, with a surface off the tool that needed almost no sanding. I set it on the bench, swept up, and went in for the evening.
It was June, the garage was about 26 °C in the afternoon and the door had been open for the breeze. The bowl lost moisture from its surface faster than the moisture inside could migrate out, the outside shrank around a core that had not, and the stress went straight through the wall at the thinnest point. The crack ran from rim to foot along a ray of the grain. When I picked it up in the morning it opened in my hands.
The rules I now follow, learned from this and from reading afterwards:
- Green wood gets rough-turned to about a tenth of its diameter in wall thickness and dried slowly, then returned to the lathe, not finished in one go
- Anything turned wet goes straight into a paper bag with shavings, or gets wax emulsion on the end grain, before I leave the garage
- No wet turning left in the open on a warm day with the door up, not even for an hour
- A thin bowl in one session from wet wood needs an even wall under 4 mm all the way round, so the whole thing can move together
The two halves sit on the shelf above the lathe as a reminder. The second green bowl, a week later, was rough-turned to 18 mm walls, bagged, and is drying now. I weigh it every Sunday and write the number on the bag; it has lost 140 g in three weeks and the rate is slowing. When two weeks pass without change it goes back on the lathe.