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Cooked tops, got potato chips
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=23725		 Page 1 of 1
Author:  David Malicky [ Mon Sep 07, 2009 5:01 pm ]
Post subject:  Cooked tops, got potato chips
I cooked 5 tops yesterday and got quite a bit of warping in almost all of them. Here are details--

- 5 StewMac AA top sets (5x2), stickered with small bricks on top, and aluminum foil + air gaps shielding the wood from heaters on top and bottom.
- Standard convection oven, 2 hrs at 200F (monitored with thermocouple in the middle of the plate stack).
- Opened door occasionally to release steam and check IR temps... typically 190-210F.
- Cooled by cracking oven door a few inches, to 90F.
- The tops showed warpage right out of the oven, which got a bit worse after stacking them overnight with moderate weights on top.
- Today, holding 3 corners flat, the worst of them is warped 2" and most are around 1". I've now stickered them with heavy weights.

Would anyone know why they warped so much, how to prevent it, and/or what I can do to flatten these 5 sets?

Thanks, David
Author:  John Lewis [ Mon Sep 07, 2009 5:26 pm ]
Post subject:  Re: Cooked tops, got potato chips
I would guess that you would only want to cook tops that are dry and already seasoned a few years. Then you would weight and sticker them and wait a month to allow them to reacclimate before building. These tops might not have been completely dry and they warped because of that. Hopefully other, more experienced, builders will come along and tell you how to get these tops flat again.

Good luck-
Author:  John Platko [ Mon Sep 07, 2009 6:16 pm ]
Post subject:  Re: Cooked tops, got potato chips
Stickering them is a good idea but it's pretty normal for me to get some warping when they come out of the oven, although you seem to have gotten a lot. It can be a bit of a shocker. It only takes a few days (3-4) for them to get back to normal. Everyboard I cooked ended up as good as new- maybe better.

John
Author:  bluescreek [ Mon Sep 07, 2009 6:50 pm ]
Post subject:  Re: Cooked tops, got potato chips
I don't agree with this at all. A good dry top is at 6to 8%. I will heat a top and back with a special heat blanket. I take it to 275 for at most 10 minutes. If you over dry the top you can create internal stress. Wood is going to go to a balance with the external RH.
Author:  truckjohn [ Mon Sep 07, 2009 7:30 pm ]
Post subject:  Re: Cooked tops, got potato chips
I would expect Stew Mac to sell stuff that is pretty thoroughly dried...
If you got it from a wood cutter that wrestled it out of the bush 2-weeks ago... different story...

Remember that you forced the MC of that wood to almost 0 from whatever it was....

And now it has to come back up from 0 to a bit below its usual EMC... probably 6% or so...

and it is absorbing water differently one 1 side than on the other... for whatever reason. My usual cause is that 1-side gets better air circulation than the other in the stack.

Give it 2-weeks sticked up with good air circulation. If you are ansy, flip them over every day... and re-sticker them up.

This sort of thing is nothing new for me.... especially when I get a fresh shipment of top wood from Way up north in the Spring... and I am down here in Humid South Carolina.... Pull those tops out of their box and they all go Twisty-Curly-Warpy for a day or two... then settle back down to flat.

Good luck

John
Author:  John Platko [ Mon Sep 07, 2009 11:52 pm ]
Post subject:  Re: Cooked tops, got potato chips
bluescreek wrote:
I don't agree with this at all. A good dry top is at 6to 8%. I will heat a top and back with a special heat blanket. I take it to 275 for at most 10 minutes. If you over dry the top you can create internal stress. Wood is going to go to a balance with the external RH.


The idea is to start with wood that is 6-8%, I guess a bit more would be ok too, and then take it down to virtually 0%. The theory, (not mine) is that this pre-shrinks the wood and when it returns to 6-8% MC it is shrunk a bit. Then the wood is a bit less sensitve to changes in RH. It does indeed seem to return to something a bit less then where it started. Another part of the theory is that the resins are hardened. The wood does seem to plane a bit differently after being "treated". It seems crispy to me but maybe that's just my imagination.

I think one luthier doesn't like to go above 180F because he noticed discoloration at higher temps. I've done some tests and I can't notice color differences at 200F but my eyes aren't the best.

I put marks on the boards before I bake so I can measure the difference and also use the marks as a guide for when the wood has recovered. Recovery is pretty fast.

John
Author:  wbergman [ Tue Sep 08, 2009 7:00 am ]
Post subject:  Re: Cooked tops, got potato chips
I only scanned the other posts, so forgive if I duplicate.

One luthier I know mentioned that he cooks wood in a press. So, you would use heating blankets and cover the the wood with a wieght (board) which you leave until toatally cooled.

It is my opinion that you should never have and residue of wax on any wood that you heat (even a hot storage shed). When wood reaches the temperature at which wax melts, some of the wax will penetrate into the wood and never come out. You may not notice it, but down the road your glue will know the difference.
Author:  TonyKarol [ Tue Sep 08, 2009 7:04 am ]
Post subject:  Re: Cooked tops, got potato chips
I cant say because I have never tried, as far as opening the oven door after baking, but I do not open the door to cool .. leave them in the oven and cool very slowly overnight. Its amazing how long it taeks for the wood to cool down - it holds heat very well.

I havent any any major warping and I have baked a few dozen tops now - 195F, convection for 2 hours. Havent seen any discolouring either. The wood is stickered and the sticker ends wrapped in wire, tight.
Author:  bluescreek [ Tue Sep 08, 2009 7:28 am ]
Post subject:  Re: Cooked tops, got potato chips
I have built over 100 guitars. I understand the theory but the fact is you cannot take wood to 0%. Just a few minutes of heat above the boiling point is all you need. This will help the excess moisture to vapor off . Just because you read it on the internet doesn't make it a fact. If you over dry the top you will have stress build up once it is braced. As the cells expand to accept moisture you can over dome the top .
Roasting the top as long as was done in this case is not helping the top at all. If you want a stable top , use one that is properly dried and stored. If the top is moving around , it isn't stable and isn't a top you want to build with .
Author:  John Platko [ Tue Sep 08, 2009 7:49 am ]
Post subject:  Re: Cooked tops, got potato chips
bluescreek wrote:
I have built over 100 guitars. I understand the theory but the fact is you cannot take wood to 0%. Just a few minutes of heat above the boiling point is all you need. This will help the excess moisture to vapor off .


The cooking I'm talking about has nothing to do with excess moisture. It's about shrinking the wood. Which is easy enough to demonstrate if you put some marks on the wood while it's stabalized at a given MC which is easy to do if you control RH. Then cook. Then let the wood return to the same MC that you started with, by putting it back in the same RH. Doing this, I find that it has in fact shrunk a bit accross the gain. If you do this with before and after measurements you can use the charts available to calcuate the moisture content after baking and it does indeed, for all practicle purpose, go down to pretty much dry.

Quote:
Just because you read it on the internet doesn't make it a fact.


I definitely agree with that.

Quote:
If you over dry the top you will have stress build up once it is braced. As the cells expand to accept moisture you can over dome the top .


Cooking has nothing to do with over dry tops that then get braced and over dome when moisture returns.

An important part of the process is to let the wood return to "normal" MC after the baking. Again, this has nothing to do with drying out wood in a permanent way.

John
Author:  grumpy [ Tue Sep 08, 2009 7:55 am ]
Post subject:  Re: Cooked tops, got potato chips
Just because you read it on the internet doesn't make it a fact

Tell Bob Taylor and Bill Collings!

Just because we don't understand or agree with something doesn't make it wrong, either.

I still bake, also. I think you went too hot, too long, too fast. No need to go right up to 200; close is good enough. One top at a time, also, unless you have the large ovens the big boys use. Sticker them, leave them be for a month to re-acclimate, and they'll be fine to use; they were indeed most likely too green. Next time, give 'em a year(luthin's about patience...) before baking, and if you doubt a top, only take it to 150-160; that will get you 99% of the way there, anyhow.

Back to deep lurkdom.....
Author:  bluescreek [ Tue Sep 08, 2009 8:11 am ]
Post subject:  Re: Cooked tops, got potato chips
That was the point grumpy , there is a difference between heating the top and roasting one isn't there . even you agree with me if you look at what you posted. Heating to near the boiling point is all you need to get excess moisture out. I still stick with the advice be reasonable this isn't a cake .
Author:  grumpy [ Tue Sep 08, 2009 8:56 am ]
Post subject:  Re: Cooked tops, got potato chips
Heating to near the boiling point is all you need to get excess moisture out

Right, and in the first post, he says he took it to 200°F. Boiling point is 212°, so he was well below, and not roasting the wood, just likely took it there too quickly for the woods' age.
Author:  bluescreek [ Tue Sep 08, 2009 12:56 pm ]
Post subject:  Re: Cooked tops, got potato chips
Wood that contains moisture reacts differently to varying
temperature than does dry wood. When moist wood is
heated, it tends to expand because of normal thermal expansion
and to shrink because of loss in moisture content. Unless
the wood is very dry initially (perhaps 3% or 4% moisture
content or less), shrinkage caused by moisture loss on
heating will be greater than thermal expansion, so the net
dimensional change on heating will be negative. Wood at
intermediate moisture levels (about 8% to 20%) will expand
when first heated, then gradually shrink to a volume smaller
than the initial volume as the wood gradually loses water
while in the heated condition.
Here is some scientific data I think we can all enjoy

http://www.fpl.fs.fed.us/documnts/fplgt ... 3/ch03.pdf
Author:  David Malicky [ Tue Sep 08, 2009 11:54 pm ]
Post subject:  Re: Cooked tops, got potato chips
Thanks, all, very much! Very helpful replies.

Yes, these tops are straight from StewMac, no additional seasoning by me, so that may have worsened things. Unfortunately I can't season the tops -- this is for a class project starting in a few weeks.

I just checked them tonight and the stickering + time seems to be working... hopefully trends will continue. I'll add a fan and check again in a few days.

That makes sense, that the warp could be due to small humidity differentials across the wood as it comes back to EMC.

Nice idea to mark and measure the boards before and after cooking, to track recovery.

I baked them at 200F for 2h because that was my understanding of what Taylor does. But sounds like 190F for 1h would be fine. I'll also keep the door closed during cool down, and ramp up to temp more slowly.

My intent for all this is to minimize in-use RH sensitivity as many of you mentioned, particularly since we build flat flat tops. My prior prototypes developed sunk tops and cracks on dry days. We'll also reduce humidity a bit before bracing for a slight dome.

I'll try another (smaller) batch with these changes. Or if the first batch flattens within a week or so, then it's all good.

Thanks again!
David
Author:  John Platko [ Wed Sep 09, 2009 9:02 am ]
Post subject:  Re: Cooked tops, got potato chips
A while back, I did a little experiment baking a top on another forum. Here's the meat of it. Since then I've decided to just use Tayor's recipe and bake for 2 hours at 200F. If you did some more baking and took a few measurements it would be great to see what kind of results you got.

6/9/08
Quote:
I baked some Engelmann today.

I baked at 190 for an hour.

The Engelmann was very tight grain. 42-22 GPI. I can't directly measure the moisture content but
I would expect it to be about 8% given the RH of my shop.

A 210 mm line across the grain shrank 2 mm. on both tops.

The thickness went from .1625" to .160" on one top and .160" to .157" on the other.

It was pretty hard to measure any lengthwise change. Maybe .5mm on a 520 mm line.

Doing some math and using a 28% fiber saturation point for Engelmann (does anyone have a better number?)
I calculate a 7% drop in moisture across the grain.

One board cupped a bit but not that much. There really wasn't any color change to speak of. I guess I'll let it pick up some moisture and see what happens.


6/11/08
Quote:
The top has had a few days to pick up moisture and it seems happy to do that.

The 210 mm line across one side of the top that lost 2 mm after baking has recovered about 1.5 mm back. The other side of the top that also lost 2 mm after baking as recovered a little over 1.mm back. This is the side of the top that curled a bit in the oven.

Most, maybe all, of the thickness that was lost in baking has recovered.


6/19/08
Quote:
It looks like the wood in my experiment stabilized with one top gaining back 1.75mm of the 2mm that it lost and the other top gaining back 1.25 mm of the 2 mm that it lost. I'm not sure why they are not the same, perhaps the 1.25 mm side was closer to the heat. It was the side that had curled a bit. Both sides have lost any curl that they had after the oven.


Just thought you might be interested. This was my first baking experience. Since then I'm a bit more careful about how I sticker and halfway through I swap boards around, I get more even results now.

John
Author:  Alexandru Marian [ Wed Sep 09, 2009 9:47 am ]
Post subject:  Re: Cooked tops, got potato chips
I wonder about using the power of the sun for a more delicate approach. At least where I live, in the summer, over 100F and 30% humidity is something normal. The tops can be taken out daily for a few hours, either sticker clamped or simply laid on the floor and turned over once or twice.
Author:  wbergman [ Thu Sep 10, 2009 11:14 am ]
Post subject:  Re: Cooked tops, got potato chips
I read something either about Torres (guitars) or Stradivari (violins) that they would spread wood out on the roof in the blazing sun periodically.
Author:  David Malicky [ Fri Sep 11, 2009 1:01 am ]
Post subject:  Re: Cooked tops, got potato chips
Update: after 2 more days stickered with the fan, they are all now reasonably flat (flat enough that their own weight pulls them flat on a table). bliss

So, 200F for 2h seems to be fine (if you're willing to wait a bit after). I imagine that would not surprise Taylor.

John, Thanks for that data -- yes, that's interesting and helpful. Someday I'd hope to do some similar measurements.

Thanks again to all,
David
Author:  Dave Stewart [ Sun Sep 13, 2009 8:32 am ]
Post subject:  Re: Cooked tops, got potato chips
Has anyone tried (or would you have strong feelings for/against) similarly cooking archtop plates after carving??
Author:  jordan aceto [ Sun Sep 13, 2009 10:41 am ]
Post subject:  Re: Cooked tops, got potato chips
Dave Stewart wrote:
Has anyone tried (or would you have strong feelings for/against) similarly cooking archtop plates after carving??


The only tricky thing is that the halves will be glued up already. I do know one archtop builder who joins the wedges with brown paper in the middle, carves the arches inside and out to fairly close to final graduations, breaks the seam open (easy and clean because of the paper) THEN re-joints the carved plates and does the final glue up. Maybe that would be a way to approach it? I won't be doing it any time soon...
Author:  Dave Stewart [ Sun Sep 13, 2009 4:42 pm ]
Post subject:  Re: Cooked tops, got potato chips
Good point Jordan... should have thought it out that far before asking! (Lord, save me from my great ideas!)
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