I think you have the right idea now regarding the temper colours left on reeds by different makers. My suggestion regarding chemical blueing was as a possible means of increasing the corrosion resistance of the upper face of the reed where the original oxide film left by tempering has been removed by filing leaving areas more vulnerable to rust attack
Not everyone's cup of tea I am sure but just in case anyone out there is curious to understand a little more about reed steel, I have added below a, hopefully correct ;-) and understandable explanation of how hardening and tempering works.
The hardening and tempering process for carbon steel (from which spring/reed steel is made) is really a method by which the crystalline structure of the steel is altered and managed in order to strike a balance between hardness and brittleness suitable for the use to which the steel will be put..
When steel is heated to cherry red colour (approx 1500deg C), the crystalline structure alters to form Austenite If, however, it is cooled suddenly by quenching in a bath of oil, a new crystal structure, martensite, is formed. This crystal form is characterized by an angular needle like structure with a very high hardness.
While martensitic steel is extremely hard, it is also extremely brittle and will break or chip with the slightest shock. Furthermore, internal stresses remain in the material from the sudden high temperature quenching and these will also tend to lead to fatigue failure. Tempering relieves these stresses and causes a partial decomposition of the martensite into other crystalline forms called ferrite & cementite. The amount of this partial phase change is controlled by the tempering temperature. The tempered steel is not as hard as pure martensite, but is much tougher.
So to produce material suitable for reeds the steel is first heated to cherry red and is then cooled very quickly in an oil bath. This gives a very hard but brittle material. In this state it would be almost impossible to file with normal tools and would have a very short life due to it’s brittleness.
The material is then heated slowly up to a tempering temperature somewhere between 450 and 600 deg C. As the steel is heated some of the martensite changes to ferrite and cementite and the hardness decreases but the material gets tougher. At any given temperature in this process, the particular crystalline nature and its degree of toughness/hardness, can be permanently fixed in place by rapid cooling in a quenching bath.
As the temperature rises, the surface of the steel undergoes some spectacular temperature specific colour changes. These are due to the formation of different complex oxides of iron which fortuitously are different in colour.
Starting at around 400 deg a straw colour appears which changes through light and dark brown, browny purple, light and dark purple and eventually at around 575 – 600 deg the familiar ‘blueing’ often seen on the underside of reeds.
In order to allow proper heat distribution and the dissipation of internal stresses, the material is held at the required temperature for a period of hours before being quenched to ‘lock’ the required crystalline structure in place
It follows that at this higher temperature, more of the Martensite is converted and the blue reed material is significantly softer than straw colour material. It will also be significantly softer and easier to work.
This would also tally with Paul’s comment that early Crabb and Jeffries reeds have a straw like colour….. they are notoriously hard and hence difficult to file.
What the exact relationships are between brittleness, hardness and ‘springiness’ or indeed how these might relate to the sound produced by a reed I cannot say.
Hardening and softening of brass can also be carried out by heat treatment. I am somewhat less sure of the details of the processes but, for what it is worth, here goes….
When brass is heated to a particular yellowy/orange colour (best viewed in semi darkness) and then allowed to cool slowly, the brass will harden significantly. ( the exact opposite to steel you may note)
If the brass is heated to this temperature but is cooled rapidly, it will anneal or ‘soften’ the material.
The process is repeatedly used during the process of making brass instruments and the like since, as brass is hammered, it becomes harder and, you’ve guessed it, more brittle. If work were not stopped periodically to anneal the brass by heat treatment, the brass would split under the hammer.
I have also heard that, sometimes, completed instruments ( Not Concertinas !!!
) are also cryogenically treated at very low temperatures to relieve and distribute the internal material stresses built up in the forming processes. This is said to give a much rounder and fuller tone to the instrument. How this works I have no idea.
If you wish to try hardening reed sized pieces of brass remember their surface area is high in comparison to their mass and they will tend to cool rapidly…. The exact opposite to what you want to achieve.
One tip is to preheat a block of steel to a similar temperature, place the heated reed on the block and cover it with some form of heatproof insulation. Rockwool mat or similar) This should allow for the desired slow cooling.
If you have made it to the bottom of this post and not dozed off, I am both flattered and impressed.
Edited by Dave Prebble, 01 December 2003 - 08:13 AM.