Ozbow

Although I have been using wet heat [steam] in my bow making for many years I recently used dry heat [heat gun] to remove the propeller twist in the limbs of a flat bow.

I was surprised at how little time it took to complete the task, probably about fifteen minutes to do both limbs.

Steam takes considerably longer, after getting a good head of steam, depending on the timber and its cross section dimensions it can take up to an hour or more for the timber to be pliable enough to bend and not raise splinters.

The following link is to a short video showing how easy it is to use dry heat and the only tools needed are a few clamps.

https://www.youtube.com/watch?v=2GerQcfYrGo

You will notice that vegetable type oil is applied to the area to aid in heat penetration.

A FEW WORDS OF WARNING; do not use OILS of any description if you have subsequent gluing applications to carry out in or near the heat treated area.

It is for this very reason I use steam when making my bamboo backed bows.

Daryl.

I can definitely vouch for using dry heat on air dried staves, never fancied the thought of using steam though...wouldn't it up the moisture content in the wood? And visa versa never fancied the idea of using dry heat on a 'brittle' kiln dried board. Would the steam help to rehydrate the kiln dried board a little and possibly help lessen it's brittleness? ...and visa versa again....the dry heat 'over dry' the air dried stave and possibly cause brittleness?

cmoore wrote:I can definitely vouch for using dry heat on air dried staves, never fancied the thought of using steam though...wouldn't it up the moisture content in the wood?

Steam may initially increase the moisture content but once out of the steamer the stave seems to dry fairly quickly, possibly from latent heat within the stave.

Generally I leave the stave for 24 hours to come into equilibrium with its surroundings before doing any more work to it.

cmoore wrote:Would the steam help to rehydrate the kiln dried board a little and possibly help lessen it's brittleness?

The last three steamed bows were made from kiln dried timber.

For want of a better word I think steam has a less aggressive affect than dry heat on timber.

cmoore wrote:.and visa versa again....the dry heat 'over dry' the air dried stave and possibly cause brittleness?

Quite possible.

Daryl.

greybeard wrote:You will notice that vegetable type oil is applied to the area to aid in heat penetration.

A FEW WORDS OF WARNING; do not use OILS of any description if you have subsequent gluing applications to carry out in or near the heat treated area.

It is for this very reason I use steam when making my bamboo backed bows.

Daryl.

I heard/read somewhere that you can use cloudy ammonia to aid in heat penetration when using a heat gun that and it doesn't effect gluing-up. I tried in on a warped piece of ply that I wanted to flatten and it seemed to work. Ponged for a while, but that had worn off by the time I'd finished heating.

In reverse order, Alan, just from the experience of one of my past lives as a Registered Nurse, I would be a bit skittish about cooking anything amoniac with my face and breathing apparatus over it, certain for repeated use.

LIke Daryl, I have done both steam and dry and would offer the same advice. More latterly, I have been using dry heat to remove the string follow in the old wood bows in my collection. These date from as early as the late 1920s up to the 1950s. I apply the heat as Daryl does using an ordinary heat gun. I have posted the process elsewhere on Ozbow, and I have not had a single failure from overheating to brittleness yet. I think I have done about 40 or 50 bows now out of my collection of over 100 old bows from the pre-fibreglass period.

The process does work. I did used to use a cooking oil to aid in heat introduction into the wood and to help prevent scorching, but found later that it was not necessary. I just needed to hold the heat gun a little further away and for a bit longer.

In my post, there is a formula for assessing the amount of reverse bend to put in a bow in order to straighten it out.

But as Daryl has done, I too would vouch for the general safety of the method. I live in a pretty low humidity area in the Southern Riverina of NSW and wood bows tend to get stronger the dryer they become, including those in my collectiion. None have failed with the application of heat to treat bending issues. If it was going to happen, it should have done so by now. But it seems that if the wood is sound, then the heat doesn't affect it badly.

Perhaps the duration of heat application and its termperature is not enough to do any damage internally or externally.

Dennis, one of the qualities of steam that I like is that if I mess up a recurve tip I can put the limb back in the steamer and the recurve will uncurl and the limb returns to its original shape.

Once this happens the limb tip can be recurved again to the desired shape.

To date I have not tried carrying out this procedure with dry heat.

Daryl.

Daryl,

I have no experience of that method. It would never have occurred to me anyway. But, I have not been much for putting recurves into bows; but that is just from personal preference. I am fond of very simple straight ended designs and getting the best I can from that design.

I do admire your ability to do the recurves however. It requires much more skill than for straight ended bows.