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Some years ago, I began cooking my tops for an hour at 200*F as a result of benefits posted by those who have done so for many years. It is believed that such time & temp cooks out much of the resin and solidifies that which remains...an accelerated aging process. The benefits are that it significantly dries out the wood in a controlled manner and renders the wood more resistant to future humidity fluctuations, particularly under dry conditions. Be aware that I only cook tops that have aged in my shop for at least 1 year...not to dry out green wood. Tops treated in this manner also seem to produce a much more sonorous tap tone than before cooking. My kitchen oven is too small for most spruce tops to fit, and as a result I would periodically use a friend's much larger oven. Well, I no longer have access to the oven so I decided to build something for the shop that would achieve the same results as the kitchen oven.

I made a box from 1/2" Baltic Birch Plywood having inside dimensions of 12"x30"x8". The top swings open using a piano hinge. It is designed to bake 2-3 stickered and weighted sets at a time. The inside is lined with aluminum roof flashing material in order to better insulate the interior and contain the heat internally. And here's the real cool feature...the heat source is my silicone heating blanket and controller that I use for bending sides. The heating blanket rests on the floor and bends about halfway up the sides. It is spaced 2" below the first board. I have a hole drilled in the side for an oven thermometer and can monitor the temperature throughout the process. I just completed some trials first to dry out the oven itself and to insure that I can reach and control the temperature. The second trial just completed was to bake 2 sets for the complete cycle.

Using 2 bricks to weigh down the stickered array, it took about an hour to achieve 200*F. I'm sure I can reduce this time by pouring the heat to it faster as well as by pre-heating the bricks. Once it achieved 200*, the controller was able to maintain +/- 5* for the remaining hour. One observation was that moisture condenses on the aluminum lining so I periodically opened the top to allow for some steam to escape. After the hour at temperature was completed, I turned off the power and allowed the oven to slowly cool with the top left open by a few inches.

The tops showed the same outward visible resin exudation and dry crisp appearance that I had seen in prior sessions and there were no signs of burning. And while more trials will be made, I'm confident that the oven is capable of baking tops in the same manner and with the same results achieved in the kitchen oven I used previously.

Check out the pics...they are self-explanatory.

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JJ
Napa, CA
http://www.DonohueGuitars.com

JJ, it looks like you really thought this out before building. I love the built-in 'slots' to help with stickering. I've wondered about getting an oven for the workshop before, but I don't have the room. I think I'm gonna build one just like yours.

Thanks for posting!

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-Clint Beacock

Very Cool JJ. I was going to do that too as our oven is too small plus my wife issued a ban on using it for anything other than food. My design would have used heat lamps. I love the idea of using the heat blanket. That should make the heat a lot more even. I did find a temperature limiting switch that opens at 200 degrees. I don't know if it would be helpful for your design or not but here's the URL.

http://www.amazon.com/gp/product/B0026R ... oh_product

Terry

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Great info on the switch, Terry...thanks!

Yep...my first inclination was to use light bulbs and this was designed to be retro-fitted with bulbs had the blanket not worked. It would have made the height considerably taller in addition to having to design for more uniform heat distribution. I just got lucky with the blanket..

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JJ
Napa, CA
http://www.DonohueGuitars.com

Great idea JJ! Might have to build one myself.

Chuck

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JJ, that rocks Bro!

Very good idea and one that I think I'll have to borrow

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What are everyone's feelings on baking old tops. I've got a bunch that were cut 30 years ago. Would it really add anything? I'm definitely going to bake newer tops.
Terry

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It's not what you don't know that hurts you, it's what you do know that's wrong.

I don't believe I have any tops that old but it should take you only 2 hours to test it. Bake one set and compare it to those that were unbaked. If you see and hear no difference then perhaps the natural aging process did its thing. I know with the younger wood, there's a perceptible difference between baked and unbaked.

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JJ
Napa, CA
http://www.DonohueGuitars.com

If cooking the tops for stability, would a guy cook the B&Sides too?

My sides get cooked when I bend them.

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JJ
Napa, CA
http://www.DonohueGuitars.com

That's pretty ingenious, JJ. As far as wear and tear goes, do you think it's ok to leave the blanket on that long?

Ingenious JJ.

I think I'll be a thieving that idea as well.

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Bob Connor
Geelong, Australia

JJ, that's wicked cool. I really like the slots for the stickers.

A student at Red Rocks made something similar a few semesters ago, although not guitar related at all. He used some kind of insulating foam, about 2" thick, to build the box out of, and lined it with aluminum foil, and heated it with light bulbs. Worked really well.

I believe Kent Everett also has something similar that he uses to quickly dry out tops just a bit right before joining them.

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Mike

"The Dude abides. I don't know about you but I take comfort in that. It's good knowin' he's out there. The Dude. Takin' 'er easy for all us sinners. Shoosh." The Stranger

Hey JJ. This is cool.

I have the same kitchen oven problem as you. I have thought about building something like this before too. However, I always worry that the tops closer to the heat source are hotter. It seems to me you want to keep the temp below the boiling point. Have you tested this set up to see how consistant the temp is at different points inside... or am I just being a big worrier?

I was sort of thinking bulbs with a heat shield / diffuser, but that seems to suffer from the same problem.

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Brock Poling
Columbus, Ohio
http://www.polingguitars.com

James...Only time will tell, won't it? I've been using the same blanket for 4 years now in bending sides but admittedly for far less time than baking tops. My kitchen toaster has far more hours on it so I don't expect the blanket to crap out in the near future.

Brock...I actually share your concerns as well. During the initial run, I was only able to monitor one spot as shown in the pic. One thing I did however was to split up a set such that one plate was 2" from the blanket while its sister plate was on the very top. There was no noticeable appearance difference after the cycle was completed. The temp was kept between 195* and 205*. I plan to map a more extensive temperature profile.

I also plan to modify the oven in 2 ways:

1) Add an aluminum diffuser spaced over the blanket.
2) Add a fan to circulate the heated air within the oven.

I was surprised to see the amount of vapor that escaped after about 20 minutes into the cycle as well as the amount of condensation that accumulated on the aluminum sides and top. I'd like to try to get rid of that vapor earlier while still maintaining the temp. Perhaps even interrupting the cycle and re-shuffling the stack might be another way to purge the vapor more quickly. Just thinking out loud here but willing to hear others thoughts as well.

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JJ
Napa, CA
http://www.DonohueGuitars.com

I had mine all designed JJ with the thermal switch and a little fan at one end and then stumbled across a nice used oven cheap so I wussed out and bought the oven and stuck it in the garage adjoining my shop. The fan I found was used with tanning beds I think and it looked just right. That might help the condensation problem.

http://cgi.ebay.com/Tanning-Bed-Part--- ... dZViewItem

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Terry...were you planning to mount this inside and only circulating hot air...or outside and drawing in fresh cold air?

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JJ
Napa, CA
http://www.DonohueGuitars.com

I was thinking about putting it at one end and having an opening at the other end to draw air through the box.
TJK

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It's not what you don't know that hurts you, it's what you do know that's wrong.

I might be taken as stupid now but---- why does one do this ? if its for taking out the moisture from the tops, wont that go back in as its then is left in the room etc ?? Is it only for newly cut tops etc ?? .

Lars

And most importantly, you now have a way to warm up that leftover right in the shop!

Joe

Right now I'm torn between how to design for air circulation. I see 2 options:

1) Mount a fan internally such that it will circulate just the hot air within the chamber like a convection oven. Convection ovens have uniform temperature distribution and efficient heating. The downside is that moisture can escape only if the door is opened periodically.

2) Mount a fan and 200* limit switch such that it draws outside air across the chamber. This will insure that vapors are removed before they condense on the sides and top. The important element here is to design for proper airflow...fan speed and port size are critical.

Are there any thermodynamic experts in the house?



Lars...it's far more than moisture removal. The baking procedure modifies the wood's cellular structure and also removes and crystallizes much of it's resin in what only can be described as an accelerated aging process. I believe it renders it more dense following baking such that it will have less change from exposure to wider variations in ambient RH. I have noticed that the tops seem stiffer and have a more pronounce tap tone following baking.

Baking is not recommended for green wood since rapid drying can cause splitting. I keep my tops in the shop for at least 1 year before baking and try to use them within 3 months after baking.

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JJ
Napa, CA
http://www.DonohueGuitars.com

To add to this.

The process of baking drives out all of the moisture in the top and submits it to the harshest conditions it can experience. This causes the wood to "shrink" (duh) but after the tops are re-acclimated to proper building humidity they never quite come back to their original size. Sort of like washing your jeans in hot water and drying them on high. This is beneficial in that once the top is glued to the rim severe drops in humidity will be less likely to cause cracks.

IIRC all of this started with Bob Taylor primarily as a way to help insure against low humidity problems.

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Brock Poling
Columbus, Ohio
http://www.polingguitars.com

Cool idea - I might have to give up the idea of using my BBQ for this.

You have to vent the air if you want to vent the moisture - I'd use a little computer fan, crack the top open and have some way for cooler air to come in from below to be heated by the blanket.

You might also think of putting in a second blanket on top to smooth out any temperature variations within the box.

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The member formerly known as erikbojerik....

Good suggestion on the heat gun, Fillipo! It's always nice to have multi-purpose tools. Thanks for chiming in!

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JJ
Napa, CA
http://www.DonohueGuitars.com

JJ - You da man!!! What a clever idea this is.