I am wanting to try some experiements with tempering wood. If I can heat the wood up to say 500 degrees and hold it there for 30 minutes or so with no oxygen what might I expect. 300 or 400 degrees just might be sufficient but I don’t know for sure yet. Regular heating methods are causing too much scorching and I am wondering if the oxygen removal might reduce scorching.
This is how you make charcoal.
Yeah, long enough and charcoal is the result. What is the point of this? You can completely dry wood at lower temperatures, it just takes time.
I knew that but I figure it is probably higher temperatures and they allow the burning of the wood to deplete the oxygen. I will be bathing in nitrogen or some other suitable gas. I just need to buy some time.
It would probably help if you gave some sort of clue as to what you were attempting to accomplish.
I make all wood primitive bows and arrows, I temper it with a heat gun right now but if I go too far I get scorching. I am hoping to get a better tempering of the wood to reduce the histerisis in the bows. I compete in flight shooting so any edge I can get helps a lot.
Thermochemical decomposition of wood begins around 400 F. This article may be of interest.Pyrolysis - Wikipedia
Wood will have air within it, so if you want a truly low level of oxygen you’ll probably need to put it under a good vacuum for an hour or so, then flood it with nitrogen.
How primitive is it when you are doing something like heating materials in a nitrogen flooded atmosphere?
I’m not going to say it can’t be done, but you can’t temper wood with dry heat. Wood is basically a fiber reinforced polymer in which the polymer is lignin and the reinforcing fibers are primarily cellulose. Both are very large organic molecules, the degree of polymerization of cellulose in wood is up there around 6,000 or so, so what you’ve got is 6,000 six-carbon glucose molecules in a chain. That varies with species of tree, whether it was early in the season or late, the age of the tree and such, but it’s not gonna soften or harden with heat. The lignin is an amorphous solid with a molecular weight around 10,000 or so. Lignin will pyrolyze before it will soften under high temperature.
Lignin will dissolve in caustic solutions given time and elevated temperatures, that’s the basis of the Kraft cooking process that makes most of the chemical grades of wood pulp, but I’m betting that’s no help to you.
I know people use moist heat, actually a steam box, to bend wood for furniture and other uses. Would that work for your bow?
I don’t think you understand the term “tempering”. If you are trying to toughen the wood (i.e. make it more resistant to wear) this is not the way to go about it. If you are trying to strengthen the wood (give it higher stiffness and develop more force for a given deflection) you’ll need to go to a different type of wood. Bill Door provides an accurate and detailed explanation for how wood is constituted and why the application of heat (and controlled cooling) will not have the same effect on wood as it does in metallic and ceramic structures where it changes the crystalline structure and distribution of atoms, or creating a distribution of internal compressive and tensile stresses to strengthen and harden the material.
Stranger
Depending on the wood we use either dry heat or steam can be used for bending. Steam is usually preffered for tight bends while dry heat is usually better for more subtle bends.
As for tempering, we use the word loosely but fire hardening of wood is a long accepted practice. For example a bow that is 45# at full draw before tempering may go to between 50# and 55# after tempering. We generaly use heat guns for the heat treating process. I am just looking to improve on it a bit where ever I can if possible.
There was some good information here.
That’s the issue, tempering is a word that apparently has several meanings. In metallurgy, tempering actually produces a metal less hard than it was. Tempering is used to increase toughness, to make it less subject to fracture. Tempering is used after hardening to reduce hardness and stiffness and make the metal less brittle. I would assume fire hardening of wood to be more or less a controlled pyrolysis reaction that changes the cross linking in the organic polymers making the wood more homogeneous and less apt to fracture along the grain.
The biggest issue we have is on the compression side of the wood. The wood tends to have some memory after flexing which slightly delays the process of returning to its original position durring the shot. Fiberglass and synthetics don’t have this problem. Heat treating by hand is effective but also somewhat hit or miss. I would like to figure out what an optimal temperature would be for more consistency.
Honeybadger, I’ve done a fair bit of work with pyrolysis and wood drying at the industrial stage. . Actually more relevant to fiber drying where wood is chopped up into chips (fibers) and dried - before getting bonded by resin to make OSB (Oriented strand board) used in housing and other application. If you have a fairly new house chances are there is a lot of OSB in your house roof, walls etc.
First obligatory warning - drying woods by thermal methods lead to volatile organics like phenol being liberated which are harmful to health - so if you are doing it repeatedly, use proper ventilation. Also - it releases a lot of resin like substances, which will gunk up systems over long times.
Now - wood chips are fairly thin and can be heated fairly rapidly to high temperatures without temperature distributions. A bow cannot be treated in the same way.
If possible, I will use a metal pipe (or better ceramic pipe) bent in the shape of the bow. I will then use an electric heating element wrapped on the pipe (after insulating the pipe) and have a thermostat to get the pipe to a desired temperature. You need a good thermostat : one that lets you select a temperature and also the heating rate. (Very important to control the heating rate - that is rise of temperature per min)
Next I will take the wooden bow and wrap some insulating wool or ceramic inserts (so that it does not touch the pipe) and insert the bow in the pipe. Preferably, I will keep the setup vertical and allow for free flow of air inside the pipe.
This is the trial and error part - but I would heat the setup upto say 300F and hold for 2 or 3 hours. I would only raise the temperature 10F per minute to say 350 and hold another 2 hours. Go in steps like this and your bow will not char.
You can have a setup to purge with CO2 (or another inert) but based on what I have seen - a well ventilated pipe will suffice.
Thats a great answer!! Your methods are similar to what I had envisioned. I will give it a try after the holidays and get back to you. What I am really curious about is whether or not I will get the same reaction from the wood without the carmelization or browning. Slight browning is no problem but I really want to avoid heavy scorching.
Honeybadger - I think you’ll have to do a few trials to arrive at the right temperature profile. I recommend junk wood pieces for trial. Wood is typically 50% water (as received) and anywhere from 50% to 70% volatile matter for the dry wood.
I would select 2 time zones - one zone around 250F-300F for drying for a long time (maybe even 24 hours) and then ramp up rapidly to say 450 for the volatiles to react and give you the bonding characteristics.
If you can apply pressure after drying - it maybe even better since it will keep the volatiles in the wood and force them to react when you take it to higher temp.
Good luck - and look forward to see the results of your experiment.
Yes, please tell us what you try and how it goes.
The actual drying I do before the heat treating although I may use the heat treating box to assist in drying. For most tropical and oily type woods I like to dy to about 8% mositure and for most of the common white woods like hickory, maple and ash for example I like about 6% for better performance. Fast drying the wood can cause severe cracks and distorsion so I usually onlu use heat drying in the latter stages of the drying process just to finish drying. My hot car in the summer seems to accomplish this pretty nicely.
I currently hold 3 world records for distance but am always looking for an edge to stretch them out a bit further. You guys here have helped me with several issues and often not even known it.