Forge Chemistry 101

[Stacy E. Apelt - Bladesmith]

Today I had to try to explain about the different types of flames to an apprentice jeweler. I showed him on paper how the ratio of fuel to air affects the weld and the metal.In welding gold,silver,and platinum it can make an enormous difference.
I thought some of you might like to see the same things as they refer to a forge running on propane.

When you burn propane, C/3H/8 , with oxygen, O/2 you will get one of three tings:
A NEUTRAL FLAME
AN OXIDIZING FLAME
A REDUCING FLAME

If the ratio is properly balanced
C/3H/8 + 5O/2 =3CO/2 + 4H/2O
Water and carbon dioxide. This is a neutral atmosphere that give the most heat with the least waste. The affect on the metal is to keep it pretty much as it is.

If the ratio is high in oxygen (more air)
C/3H/8 + 6O/2 = 3CO/2 + O/2 + 4H/2O
There is excess available oxygen in the atmosphere that will combine with any available element,like red hot iron,chromium,carbon,etc. This will create pitting, oxides,decarb,and several other undesirables. It can alter the alloy of the steel.

If there is a lack of oxygen (low air volume)
2C/3H/8 + 11O/2 = 6CO + 16H/2O
The lack of oxygen creates carbon monoxide and water. This will allow the hot gasses to combine with any available oxygen,such as the oxides on the metal,leaving a cleaner surface. It will also keep the iron and carbon in the steel from oxidizing,thus preventing scale and decarb.

For general work you want a neutral atmosphere. For welding a slightly reducing atmosphere.For melting or attaining the maximum temperatures, an oxidizing atmosphere.( in general ,you never really want an oxidizing atmosphere when making knives)

Most forges are set with a slightly reducing atmosphere. The thing to keep in mind is what is happening as you turn up/down the blower or the gas.

For some of us it has been a few years since High School Chemistry.But remember when the teacher told you,"Some day you are going to need to know this stuff." Well..............It couldn't hurt.
Stacy

PS: Wish I knew how to use subscripts and superscripts on the computer.Must be one of these buttons?

 

[docdredd]

bravo! thanks for the lesson :>

 

[A C Richards]

Thanks Stacy,

i always like the info you provide, very useful and very applicable.

When i added the thermocouple to my forge (just to let me know the temp) it made a huge difference. A small adjustment can make a drastic change.

Chuck

Chuck

 

[mete]

In a typical propane torch flame you have an inner[pale blue] and outer [darker blue] cone .The inner cone is reducing , the outer cone is oxidizing and the tip of the inner cone is the hottest part of the flame. That should be remembered when soldering and brazing. Remember this equation , Fe +O2 >FeO [oxide or scale !!]

 

[DAMNENG]

Tunning a forge?

Stacy

Not trying to steal your fire (no pun inteneded!)

After Satcy's excellent description of the three flame states, I assume someone is going to ask the obvious qustion --- How do I know what is going on by looking at my forge?

Here is what I go by, it's not the only method but it works for me. If you have a vertical forge ( cavity runs up and down and it has windows in the sides for inserting items to be heated)

Stand where you can view one of the ports from the side,
>if you have one or two inches of yellow flame frequently licking out of the port you are just on the reducing side of neutral
>if the yellow flames are always visible or they extend out past 3 inches frequently then you are definitely reducing
>to get the hottest neutral flame requires being able to see where the burner flame meets the wall of the forge. Start from reducing and increase air or reduce gas until the external yellow flame disappears, now watch the spot where the flame meets the wall. When the mix goes oxidizing, you will see a shadow start to form on the wall. Increase gas or reduce air until this shadow disappears and you should be neutral. The heat should rise rapidly at this point.

A similiar process works for a horizontal forge, but they usually have one large open hole so external wind currents affect how far the yellow flame extends. They also run more reducing toward the back where air doesn't enter from the opening.

Jim Arbuckle