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FAIL - Removing a warp from horn with dry heat

RumpleBearskin

Contributor
First the admission: I failed miserably at trying to remove a warp from a horn blank using dry heat.

The failing process:
Using a countertop toaster oven I heated a badly warped horn blank to about 325 degrees Fahrenheit for about 20 minutes. It showed no signs of pliability, so I raised the temp to 350 and left it for 30 minutes. When I came back it was significantly blistered, but I put it between angle Irons in a bench vice and cranked down. As I tightened it I heard several cracks.

After cooling overnight I took it out of the jig. It was flat as a pancake, but horribly blistered. I attempted to sand off the blistered surface, but the horn underneath appears to be ruined as well.

Pictures (so you can have a good laugh):
PXL_20210602_233949363.jpg PXL_20210602_233958113.jpg

So, the fried blank now belongs to my artiste wife who thinks it's the coolest thing since sliced bread.

Advice on where I went wrong is greatly appreciated... Too hot? Too long? Too hot and long? Leave it to the professionals?
 

Herrenberg

Contributor
Thank you for this. Reporting failed results goes against our instincts, but they are just as useful to readers as successful ones. Wish the scientific community would get its incentives aligned to surface failed results too.

One of my favorite quotes is from Thomas Edison. A reporter asked him about how he felt about making no progress toward a working light bulb. He responded that, on the contrary, he had made tremendous progress, because he now knew 40,000 materials that you couldn't use as a filament.
 
I have limited experience with this, but I would say too hot and too much pressure.

My most successful attempt involved putting a pizza stone, a granite tile, a pot lid and some blanks into an oven and then heating the oven to 250F. (Actually I think I started without the blanks to give the larger pieces a head start on getting up to temperature). After about 20 minutes I rearranged things so that the blanks were sandwiched between the pizza stone and the granite tile with the pot lid on top of the tile to apply more pressure. Then I let the oven go another 20 minutes. After that I turned off the heat and let everything cool down to room temperature before disassembling.

So the differences
My max temperature was lower (I think I read in other forums not to get too hot (> 300F?).
I let everything come up to temperature before applying pressure.
I applied pressure with surfaces that were at the same temperature as the blanks. My concern here is that the bench vise has a lot more thermal mass then blanks. Sort of like if you put a really hot metal spoon in a glass of water - the spoon temperature drops rapidly but the water temperature doesn’t rise very quickly.
I wasn’t really pressing that hard (in fairness my blanks were only 1/8” thick, so I didn’t need a ton of pressure.

End result
blanks did get noticeably flatter, but not completely flat
No damage to anything
Most importantly there was no smell from having hot horn in the oven!
 
I’m sorry! That sucks. I blistered one and learned my lesson. If I do the toaster oven thing I do it between two plates of ¼” steel and bring to temp for longer and then let it all cool naturally between the plates.

But seriously, a heat gun and good thick gloves are so much easier and faster. You can tweak it however you want in your hands and then a little cold water and it is fixed in that position. Heat the side it is curved toward and watch it bend outward.

Better luck next time!
 
Oh, and so you know. Once you cause delamination of the horn and it “blistered” there is no putting it back together :) Maybe some heroic efforts with epoxy etc but it will not be naturally rebonding again.
 
I use steam and clamp to flat surfaces. I might use a blow drier to heat said surfaces.
Sometimes, it takes a lot of cycles to get 90% of the twist out.
I just boil a pot of water and use a chop stick to hold the horn over the water. If it's a slab I might consider using pliers or something.
Sometimes, that stuff is just too darn jacked up to 'save it'. Not worth it to me, materials that fight me often wind up in the circular file cabinet.
 

RumpleBearskin

Contributor
Thanks for the suggestions thus far.

I have it another go a coyote of days ago. Lower heat (225 F) and less time (25 minutes with no preheat). My clamps are 4x6 angle iron so can't fit them in the oven.

Tried 4 blanks in two batches. The first batch lost some of the bow, but also picked up a twist and a ripple. Not useable IMHO. The other two flattened to what may be useable. So mixed results overall. At least nothing burned/delaminated this time.

I've ordered some 4" bar stock to try to heat the next test with some pressure in the oven, and also to retain some heat and prolong the cooling cycle.
 
I think the metal plates in the oven help more with the even heating and then even cooling that helps prevent the blistering much more than they are to clamp it flat.

Once you get horn hot enough to flex proper just one time in your hands you will understand how soft and rubbery it becomes and clamping is not important - you can bend any which way you want. It will be fixed in the position in which it cools.

This used to be a trade of professional craftsmen called horn pressers. Mostly lost like many obsolete crafts. I tried boiled/steamed which softened enough to tease me. I did the ovens with and without plates and learned lessons and finally figured that out but it was a long process bringing up to temp and cooling. Then I saw horn pressers making spoons and forks and bowls right in front of my eyes in minutes or seconds and finally tried what I had read about using a heat gun. Wish I had started there in hindsight. Lots of ways to skin cats so anything that works….works. If what you are trying continues to not work or not work to your satisfaction, then consider trying what the pros use which is dry heat. Some use flame, some use hot pressers that are essentially like molded electric irons, and some use a simple old heat gun which I found does an amazing job. It is amazing how fast and well it works.

Good luck!
 
My opinion. This post below on the topic is one of the best on this site. You read and see several ways people do this successfully. Really good info in there. I have already said what has worked and what has worked best for me.

 

RumpleBearskin

Contributor
@LJS, thank you! The heat gun is already in my Amazon cart. Just trying to see what I can accomplish with what I have. (Well, almost what I have since I bought the bar stock.) Thanks for the link. I'll be all over it this evening.
 
I have had the best luck waiting until after the rough cut of the scales. The less material I'm trying to manipulate, the better the results. Soak overnight in neatsfoot oil, 10 minutes in 200 deg. toaster oven, bend/straighten by hand and cool with water from the tap. YMMV
 
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