Steam-Bending Kiln-Dried Purpleheart
Purpleheart is one of the most difficult woods to bend, especially after it's kiln-dried. But here are some useful tips on how to best do it. September 10, 2007
Question
I need to bend some purpleheart around a 15 foot radius. The material I have available is already kiln dried. The finished material dimension will be about 3/4 x 3. I've only bent wood once. I stuck a small piece of kiln dried red oak (1/8 X 3/4) into a PVC pipe, then put it over the end of a tea kettle. It came out like a piece of spaghetti. It was very pliable.
My plan on this one is to use a wallpaper steamer. My thoughts are that about 15lbs of pressure might inject the steam in deep enough to overcome the thickness of the material. Does anybody have any other ideas how to do this? Can anybody direct me to more information about bending purpleheart?
Forum Responses
(Furniture Making Forum)
From contributor G:
We have found when steam bending kiln dried woods that, if you soak them in water overnight to get the moisture content up before steaming, it worked much better. I have not bent purple heart.
From the original questioenr:
Thanks! I hadn't considered soaking the wood.
From contributor K:
Leevalley.com has everything you will need, and the most important thing is the little free book that comes with the straps. You can even find a free download in their book section. It is only about 20 pages long, but that is more than you are going to get from a forum. As for the water, I can't see any benefit to soaking. It is not the water that makes it bend, it is the heat which softens the lignin between the cells. It is fortunate that occurs at about 212º which makes the steam a good heat source. You can type 05F15.01 in the product search for a free download book.
From contributor G:
Kiln dried lumber is not the best for steam bending but was all that was available to us when we were bending up a run of chair backs. We found they would fracture during bending without soaking, but bent well after soaking. Also, your radius is very shallow, so you can expect a lot of spring back on the pieces. You will probably have to re-work your form after the first attempt.
From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
With a 15' radius, you do not need to get the wood soaking wet and would not want to, either. Unfortunately, purpleheart is very high in density and moisture does not move easily within the wood. So, soaking may allow the outer fibers to increase moisture, but getting the core wetted will be extremely difficult. You need to use very hot water, but my choice if you have the time would be to put the wood in a steam sauna room for a week. Then steam it just before bending.
You will also find that this wood will develop some color variations if you soak it in water. Is there any chance you can trade this KD lumber for some that is only air-dried? Is the thickness of the lumber 3/4"? If it is much thicker, you will indeed have some problems. All in all, this wood is extremely hard to bend (harder than any US species) and the fact that it has been kiln dried (and maybe the brittleness increased as well) makes this tougher than...
From contributor X:
I've made my steamer out of stove pipes which were screwed together. They had end caps on them that had holes to allow for the steam to come into and escape. Inside the pipes I had racks inserted to keep my steamed wood floating like, so the steam could reach all sides of the wood - worked great.
From the original questioner:
Thanks to all of you guys! I appreciate this information a lot.
From contributor L:
Using steam pressure above 7 PSI will not benefit you. Besides increasing the potential danger, higher pressure steam will leave your stock appearing drier and less pliable.
Your difficulty with such a large radius bend in a very dense wood may be more in maintaining the shape after it's bent. Hopefully in the final use it will be secured to prevent spring back. If this is not the case, you will need to overbend and dry at 140 degrees Fahrenheit for 24 to 48 hours. It’s not impossible; I’ve bent 3/4" rosewood to a 4" inside radius, very hard on the bending straps.
From contributor Y:
Could you cut it into thin strips, put some glue on it, put it in your form, clamp it up and be done? Get your glue to the right color or a darker color. Done right, you won't see the seams. I think it is a lot easier than the bending process, and you won't get the spring back problems. Of course you have to have enough wood for the kerf cuts.
From contributor T:
I also would resaw and laminate. I have done this many times with all kinds of timber with very good results. But you need at least 3 lams to hold a curve. That is at least 2 glue joints. Resaw and grind down to .25 inch with wide belt or drum sander. Clamp tight with outer bands of hardwood to equalize pressure and joints will not show.
From contributor C:
I have also heard that if you soak the wood in a solution of water/fabric softener, this allows the wood to bend more easily. Recommended time is anywhere from 3 days to a week. Won't hurt it, just need to dry it out after bending.
