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I have built a steambox as shown above, out of plywood. I'm trying to bend some Pinus elliotii (slash pine) as a test with it, but it has not been effective.

I have left it on for thirty minutes with the steamer on, than I need to refill it, and than I leave it for another thirty minutes. The wood has not been soaked before. I'm guessing this interval of a couple of minutes (the steamer needs to reheat) could be enough for a temperature drop, or not? It seems to me refilling the water is an issue with most forms of steam generation.

Also, my box has two 1/4" holes on the bottom and one on the top for pressure release, but probably the most leaky part of the box is the lid. Should I seal any or all of these?

  • From what I've seen of these do seal it well other than the deliberate vent holes, it can't hurt (although I have seen some leaky setups that apparently are working fine!). Soak first? Well it's worth a try. One thing I notice and I don't know if it's anything significant, but the steam is going in at the side. Most boxes seem to have the steam source at one end and the major venting at the other end. This may be a necessary feature, so the steam is consistently passing over the wood, I don't know. – Graphus Oct 30 '19 at 8:05
  • BTW, presume you're aware that softwoods in general aren't a great candidate for steam bending? – Graphus Oct 30 '19 at 8:06
  • Does this box have vent holes? This is one of those perhaps unobvious things, but it is necessary if you want to maximize the steaming. Similar to how an oven works, you need a place for the energy to escape if you want to add more energy. – jdv Oct 30 '19 at 20:45
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    I am not experienced with using a steam box, but my first impression is that the the box might work better if it was oriented vertically with steam introduced at the bottom. A small pressure hole at the top would show if steam were reaching the top. Another small drain hole at the base would allow condensation to collect and drain as it cools. – Ashlar Oct 31 '19 at 0:40
  • I have seen B vent (double wall flue pipe for gas equipment) it will provide some inhalation effect. – Monte Glover Nov 12 '19 at 2:04
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I built a much smaller plywood steam box a while back using that same Karcher unit.

I used it to bend oak, ash, pine and small branches, mostly very well, after steaming them for about 20 minutes each. (See oak test below). I was using 5-6mm strips about 400mm long. I'm guessing you're working with thicker stock? In this case you'll probably need to leave it for more than half an hour.

A few pointers for how you might get better results and reduce steaming time:

  • The internal volume is too large for that small steamer. (see picture for mine). Try steaming pieces in smaller batches, in a container that fits snugly around the pieces. Use small wedges along the strip to elevate the strips from the bottom, and get steam flowing around all its surfaces.

  • As mentioned by the comment above, the steam port is in the middle. Try running the steam parallel to the wood strips, by putting the port at one of those ends. I reckon the end grain exposed at either end is the most porous surface, and it should give you better flow too. I loosely sealed the other side to vent the flowing steam.

  • Also mentioned by a comment above, softwoods don't steam-bend very well. I found pine consistently failed alongside hardwoods like ash and oak. It tended to stay rigid, or break when bent sharply. Oak and ash are ideal woods. You want pieces with as straight a grain as possible.

You have to bend the material immediately after removing it from the steam box. It very quickly loses the ability to bend as it cools and dries.

I have never tried bending thicker stock, but you may have to laminate thinner pieces if you still can't get the results you're after.

Oak test

Steam box

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