Why did it crack?

[SimonMast]

Hi all. Been a while...

Some of you might remember that I made a cup some months ago from a piece of silver birch. I've been enjoying my drams from it ever since. On Wednesday I had a tipple from it, and afterwards I put water in it and left it a few minutes (no particular reason why), before tipping it out and then putting it on the draining board; as I always do. I then went out for an hour or so, and when I returned it had developed a large crack all the way down the bowl.

Any ideas why? I have just moved house and the old one had a bit of a damp problem whereas this one very much seems not to, but is that enough to cause this? As for the cup itself, it does have pretty thick walls, but I've had no trouble before now. It's a shame, but I'm more hoping to avoid such problems in the future. My new residence has some very viable work space (wife very happy), so I'm hoping to do some more work soon...

Will put a picture up later.

 

[dwardo]

The only reason i can think of is the same for any wood. Even or uneven rapid moisture exchange causing one area to swell and one not causing cracks.
Having only played with a few cups and hardly using them i wonder how well a walnut oil finish can protect again rapid moisture transfer?
I try to avoid this as much as i can bow making by keeping wood dry and sealing the finished work as best as i can. Not to mention RH meters and accurate scales. Nothing will actually stop the process of moisture exchange but you can slow it.

We need a batman style light for Robin wood

 

[Exbomz]

Was it fully dry before/when making it? How wet did it get when you "left it a few minutes" and all the other factors in wood drying - it's not an exact science. Too fast drying and it will not be able to cope. If it has thicker walls, then that could be the problem, as it is harder to dry evenly; the outside dries qucikly, the inside is still wet and you have the conditions for splits. Thin walls and you have a better chance that the wood drying will be more even.

IMHO of course.

 

[SimonMast]

The only reason i can think of is the same for any wood. Even or uneven rapid moisture exchange causing one area to swell and one not causing cracks.
Having only played with a few cups and hardly using them i wonder how well a walnut oil finish can protect again rapid moisture transfer?
I try to avoid this as much as i can bow making by keeping wood dry and sealing the finished work as best as i can. Not to mention RH meters and accurate scales. Nothing will actually stop the process of moisture exchange but you can slow it.

We need a batman style light for Robin wood

We certainly do. I was hoping he'd offer his wisdom...

But perhaps that's it I suppose. I had wondered, but generally I don't see this kind of thing come up much (on places like this) regarding finished items. Drink from it (and some people make hot drinks in cups with oil finishes), wash it out, leave it to dry; and up until now as said that's served me just fine.

 

[SimonMast]

Was it fully dry before/when making it? How wet did it get when you "left it a few minutes" and all the other factors in wood drying - it's not an exact science. Too fast drying and it will not be able to cope. If it has thicker walls, then that could be the problem, as it is harder to dry evenly; the outside dries qucikly, the inside is still wet and you have the conditions for splits. Thin walls and you have a better chance that the wood drying will be more even.

IMHO of course.

I made the cup in about February this year having picked up the wood on Boxing Day and I kept it (in the absence of an alternative!) in my living room, so it was pretty dry when I made it. When I left it for a few minutes I filled it with water, which I'm now guessing I shouldn't have...

But thanks for the info; useful to know. I seem to remember seeing Uncle Ray with a kuksa on one of his DVDs (I think it was the one in Sweden with Lars Falt?) and the walls looked paper thin; I'm guessing that might be why...

 

[Paulm]

Haven't made a wooden cup before, but I do a fair bit of woodturning, mainly bowls, and expect a lot of the principles are the same.

As others have said, an even wall and base thickness is important, differential thicknesses will cause tension/reaction between different parts of the cup, increased by getting wet and drying out at different rates, and increasing the risk of splits.

Again as said, thinner is generally better, within reason, but if everything else is okay then this of itself is not going to be that important, unless the timber is very green when you are carving it, when thin is more important.

One of the most important things that I don't think has been mentioned, is the piece of timber used, what kind of wood, which bit of the tree it came from, grain orientation, features or faults in the wood etc. Branchwood for example will have the fibres in tension on the top of the branch as it grows and resists gravity pulling it down, while the underside of the same branch will be in compression as it pushes up against gravity. Each piece of that branch when cut will behave differently in seasoning and in use, and if you use a piece with both top and bottom growth in it then you are likely to get all sorts of problems with movement, twisting, cracking etc. Likewise, knots can often be the source of splits starting.

So lots to think about and quite a big subject, but generally if you use fairly straight grained, clean (knot free) main trunk wood, and carve it to an even thickness and not too thick, then you should have a good success rate, but of course many of the more attractive pieces of timber are the ones with features and swirling grain etc so you may have to risk a higher failure rate with those but end up with a more special result.

Species of timber has an effect too, some being more prone to cracking/splitting like sweet chestnut, cherry, willow etc, but then there are no hard and fast rules and lots of exceptions to the rule, that's part of the fun !

Cheers, Paul

 

[robin wood]

can you post a pic? I think this is a useful post as it shows that even if you dry something slowly and manage to get it dry and finished without cracks once you start to use it there can be problems.

The important things have mostly been mentioned.
1 thin even wall thickness as a ball park figure 5mm on something small max 10mm on larger things.
2 make sure the centre of the tree is not in what you make, if it is then it will split in use nearly every time. So first thing to do with your log is split it in half and start carving out of one half only.
3 finish with an oil that cures (linseed, tung, walnut) this stops the wood from absorbing lots of moisture in use, it's when it looses that moisture again that you get the cracks.