Heat Curing / Tempering Bamboo

[Mark Meckes]

When sugar is heated to a certain temperature, it loses it's elasticity and becomes brittle. This is observed when boiling maple syrup to a temperature where it becomes maple candy. The same thing happens with cane sugar.
I was discussing this with a friend who makes musical instruments made from long hollow bamboo tubes.(Phyllostachys rubromarginata etc) approx 6cm dia X 1-2 meters, (2"dia X 3-6ft )
The method he uses to temper the bamboo is to heat it when it is green, with a propane torch, moving the flame up and down the surface of the culm until it changes from shades of green to tan and if desired, to darker shades of brown. He rubs a block of bees wax on the surface during the heating process, which melts, and helps to wiped off excess wax and grime.. (see post#4)

We were wondering why very few of the culms split using this procedure...
One thought is that when the heated bamboo is cooled, the molecular change of the `sugary juices' occurs, which upon drying, `fixes` onto the cell walls, like a hard but pliable glue-like substance.

I'm sure there must be a critical temperature at which point too much heat would cause the sugars to become brittle.
Would anyone know if any research has been done on affects of heating bamboo and `sugars' within ?
Or suggestions as to what the best temperature would before heat tempering bamboo. ?

Mark

[Mark Meckes]

Derick Calderon wrote:
"heating with propane is a good solution but it changes the color of the bambu."
------------------------
This is a very specialized process and must be done on green or slightly dried bamboo.
Sometimes (but not always) the node stop, or diaphragms are first punched out with a steel bar.
The moisture in the bamboo prevents the inner wood from getting burnt.
Only a small percentage of moisture is released during this process.
Only the outer surface turns tan, gold or shades of brown, depending on the intent.
Skill must be acquired in use of the propane torch, and techniques to ensure an even distribution of heat.
With practice one is able to provide an even or mottled finish that is from tan to a rich deep brown, similar to the colour of Phyllostachys nigra. The finished product is very beautiful and much admired.

This process does not totally dry the bamboo...it may take several months or more of natural air-drying..
But the above process does heat the bamboo considerably within, similar to steam-heating, affecting the sugars in the bamboo, and I'm presuming,
condensing it to `glue'.

Mark

[BambuBrasil]

I have heard many different things about curing bamboos with direct heat using propane torches. According to the people at PUC in Rio, where they have a small lab and work with bamboo 24/7, they suggest 2 stages.
First, the bamboos should be "lightly" cured after they are harvested. They shouldn't be completely darkened or yellowed, but just lightly toasted, so you can still see some of the green (Phylostachys aurea). Then, as you use up the bamboo, you individually heat cure each one.

Something else you didn't mention was the fact, that you should always heat up the bamboo in the direction of growth. We call this in Brazil, "combing the fibers". It may sound silly and without scientific basis, but I have seen the difference myself in Belo Horizonte [Minas Gerais].
I learnt this technique from Lúcio Ventana, who has worked with bamboo for over 20 years there, and who constantly promots his "Civilização do Bambu" course throughout the state for the past 10 years.

Post Scriptum - From what I have heard and seen (once), flash-curing seems to crack at least 25% of the bamboos or more. I think when working with bamboo we have to be patient like the Japanese, and respect the plant and know it's limitations.. everything that is worthwhile and
well-done takes time!

Ciao - JP

[Mark Meckes]

The following 2 photos show Stan Skov heat tempering Phyllostachys rubromarginata, in preparation for making bamboo musical instruments:


Heating the bamboo - A propane gas flame is rapidly moved up and down the recently harvested culm until the surface waxes begin to bubble.


Applying beeswax - which acts as a solvent, aids wiping off bamboo exudation The bees wax also insulates the bamboo, slowing the rate that the outer surface of the bamboo turns brown. It's like a paint extender, enabling one to apply layers of tan, becoming browner with each stroke of the flame...

Bamboo Lebaphone - Stan Skov Technicallly called a `plosive percussive aerophone', Stan calls this instrument a Lebaphone, in tribute to a good friend. ...is also called a paddlephone and undoubtably has many other names!


Rubro Jaw Harp - Mark Meckes This is one of my jaw harps made from a heat tempered off-cut that Stan gave me.
I scraped off the browned surface coat of the bamboo to reveal a golden tan surface with a deep lustre.

Mark

[Henry Lee]

I harvested a few P. pubescens(moso) last week. I heat treated them with a plumber's propane torch till the green turned yellowish brown. I didn't let the wax bubble nor did I use any beeswax. Then I left them out under the sun for the last 5 days. Tell me if I did it wrong.

See larger pic See larger pic

[Mark Meckes]

Hi Henry, Thanks for showing the photos!
Pictures speak a thousand words.

> Tell me if I did it wrong.

There's no such thing as wrong if the final result is satisfactory to your wishes.
The truth is that there are many ways to dry and cure bamboo, though there are many factors that affect the final result.

The heating process you used does seem to temper or strengthen the bamboo, perhaps by using heat to relax or induce equilibrium of tensions among fibers within the bamboo.
When the heated bamboo cools, the outer surface of the bamboo forms a hardened `shell'.
This pic shows the different rates of contraction of a piece of green heat tempered bamboo, after it was cut into 2 inch wide strips, then air/shade dried.
See larger pic

> How long, slow or fast, to dry the bamboo after heat tempering?

Tough question! A Moso flooring manufacture, or someone drying split pieces would have a different answer then a flute maker or pole supplier.
Each bamboo piece may have it's own moisture level, because of the species, age, where grown and harvest time.

Some say 2- 3 months drying time, others 9 months to 3 years, not in direct sunlight.

Regarding sun curing after heat tempering.

I don't have a direct answer for this. I usually put my flame heat treated pieces indoors/under shade, to dry after the first day. I don't make flutes.

I do like the look of sun cured bamboo and may experiment further.

Here's some thoughts about sun drying (solar curing) bamboo ...

Bamboo is often `bleached' and cured by drying in the sun alone. It can take 2 weeks to 2 months to reach a desired finish. Much faster with shorter pieces.
Satisfactory results are dependent on the weather, day and night temperature, hours of direct sunlight, humidity, and especially rotating the culms regularly to ensure even drying.

Uneven sun exposure can force drying fibers to contract at differing rates. This could be a cause for cracks to appear later.

If the bamboo is exposed to too much heat or exposed to sun and fluctuating weather for too long, the natural glues within the bamboo can become brittle, lose their elasticity, which can lead to cracking, either as tiny fissures or large cracks.

Mark

[cycle.daedalus]

Hi everyone,

I have been reading the forums for a while and just joined since this thread is so interesting.

I have been working for the past year on developing a bamboo framed bicycle, much like the ones Calfee produces but for substantially cheaper.

We have built 6 frames so far and have had good success but are always looking to improve our design.

We have been using black bamboo canes which are timbered in China and imported to us in Portland, OR. I am wondering if heat tempering this material would be a good idea since it comes to us in a fairly dry state?

Does anyone have any experience in tempering dry canes? Any ideas? Small cracks have developed in many of the frames, I always looked at it like timber checking which is ok but building multiple redundancies in our frames is extremely important.

Let me know what you guys think.

Thanks,

Liakos

[Mark Meckes]

Hi Liakos,
I did a google search on bamboo bicycle and found some interesting links, though found little info on intrinsic details of bamboo construction, which we could address in a new thread.
I would love to see pics of your works in progress and feel free to upload pics in one of the galleries or at the forums.

Regarding heat treating after the bamboo has been thoroughly dried, again this would depend on the drying procedure, and when we think of heat tempering or hardening of the bamboo, this can imply different things.
For example heat treating or burnishing bamboo can result in harder bamboo, as anyone who has used a stick as a poker for a fireplace will attest. However heat treatment results in minute shrinkage and with a round piece of very dry bamboo this could lead to a widening of a fissure or release of tension at a weak point.

My personal experience is that freshly harvested bamboo, or bamboo that is partially dried is more malleable to heat treatment as the fibers within are not so 'locked' in place, and the moisture within the bamboo enables more heat to be applied without charring the bamboo.
I think of the surface treatment acting similarly to what a hardened eggshell does for an egg, while the heating of the inner liquids might 'fix' the molecules upon cooling.
The objective, in this form of heat tempering is that only a small percentage of moisture is displaced during this process, with the balance being dissipated through natural air drying.

Regarding Phyllostachys nigra and species ... I have found, particularly with larger diameters, that they can be susceptible to cracking with natural air drying, and are possibly easier to split (when one desires to do this) then some other Phyllostachys species.
This is just a personal observation and not totally culmclusive.

Are the cracks or fissures minute, for example around the node or is there a singular crack forming along the internode portion?

Mark

[cycle.daedalus]

Most of the fissures are minute, hairline cracks that form at the node and often extend between two nodes. We have a high strength epoxy that we use to seal these fissures and prevent water from entering.

On one frame however, before stress was ever placed on it, a massive crack appeared the whole length of the cane. This crack was about 2mm across.

From what I understand, the bamboo we are using receives months of treatment before ever being loaded in a container and shipped to seattle, where we get it from Bamboohardwoods.

We search through piles of bamboo, discarding pieces that are already cracked and choose only the straightest, nicest canes.

We are fairly content with the bamboo we are getting and feel that between our quality control and the time it takes to get it turned into a bike frame (probably 2-3 years) any cracks that would have formed will all ready be there.

One thing we will start doing is removing all the nodes from the bamboo canes, but I am wondering whether a light heat treatment and oiling would constitute a 'vapor barrier'? Thus, stopping water from enter and exiting the bamboo.

I will eventually post some pics on this site but am working to get my own website up and running.

We are really aiming to produce an open source bamboo bike and would love to get more people involved.

Thanks again,

Liakos

POST SCRIPT

I did some sample heat tempering today on my "dried" canes. There was steam coming out of the top, which means that residual water is still inside.

The tempering also darkened our bamboo (logically). Brought a waxy film to the surface (anybody know what that is?)

I used a MAPP gas torch, was very hot and concentrated heat. I think I will get a propane torch.

[Kalle Knast]

Hi Liakos,
I'm interested in building a bamboo bike, and have slowly started process of making one. I will follow Brano Meres example and build a bamboo bike with carbon joints. But, right now, I just started on the jig. Why did you chose Phyllostachys nigra? Is that the same as Calfee uses? Brano did not know which species he used. He did not treat the bamboo, but it has lasted fine. No cracks. Does anyone know where to get advice about which bamboo to chose for something like a bike? Which are the species of bamboo that do not crack? Do different species with similar diameter have different strengths?
All advice are appreciated.

Thanks,
hjalmar