On Trough Forges...

Posted by STAG to Hammered out bits at 1/08/2007

"At the 7 year's war rendevous at Fort La Presentation this last summer, the blacksmith used traditional "great" bellows, but a bit smaller than usual for ease of traveling. The forge was a wooden box, lined with clay. He burned coal, not charcoal.

"This is the simplest forge I have seen for making the long, narrow, knifemaking heat.

http://64.176.180.203/washtubforge.htm

"This is designed specifically for charcoal, and varies in many respects from a normal forge. For one thing, there is a huge area below the air source. I would be interested in the physics of why this would be better than air from below. "

A couple of things:

On historic forges:

The simplest forge is nothing more that a tube into a hole on the ground. I actually worked off a 'pit forge' for the first year or so when I started blacksmithing. I took an old cast iron hibatchi and set it into the ground for the fire box. A vacumn cleaner for the blower. Burning anthracite sweepings for the fuel (if you can believe it). A piece of rail track for the anvil.

at Heffenreffer Museum, Bristol RI, 2006
photo by Carolyn Taylor (off the DARC web site)

For Viking Age presentations I use a 'sand table' forge. Norse forges use a stone block with a whole about 2 or 3 inches up one face through which the air is pumped. The charcoal fuel is simply piled against this block. On the (speculative) reconstruction, seen above, a simple layer of sand about 3 inches deep contains the fire and protects the wooden planking of the table.

On the Lively knife making forge:

I think you are looking at the top photo on his web site - the actual metalwork details, rather than the middle photo - which shows the fire clay (adobe mix) in place. If you look at that middle photo you will see that the clay creates a slope sided containment which ends up exposing the top surface of the pipe. Rather a lot of clay used in this configuration - which has to make the whole thing darn heavy.

Direct from the Lively Forge website

This is a pretty standard arrangement for a what I've seen called a 'pipe forge' or a 'trough forge'. Reguardless of the name, the concept is to create a long thin fire through a series of smaller diameter holes running down the length of a section of tube. I have used a temporary arrangement in the distant past for the same purpose. I just dug a trench and lined it with fire brick laid down either side of the pipe. (These days I have a specially constructed propane forge with three burners in a line. This gives me effectively a 24' long firebox for heat treating swords or heating larger pieces for architectural work.)

Now I found one problem with the simple 'holes in a line' arrangement seen on the Lively forge can be seen in the image above (direct from his web site). You will notice that the fire is both largest and brightest at the blower end of the trough. In fact the amount of heat drops quite obviously towards the furthest half of the pipe tuyere.
The physics here is that the air pressure at hole one (closest to the blower) is at maximum from the blower. At hole two however, the available pressure is at blower minus whatever was pushed out of hole one. At hole three what is available is B-1-2, and so on at each hole in a progressive loss. By the time you get to about hole 4 there is hardly any air left to insert into the fire.
Ideally what you want to do is steadily INCREASE the diameter of each hole as you work down the series from the blower end onwards. A smarter guy than me could likely work the math here - its going to be some relationship between pipe volume and the area of all previous holes.

To be truthful, this forge set up is really just a waste of fuel. The only time you need to heat the ENTIRE length of a blade is when you are undertaking the heat treating, and only high temperatures are required for the hardening (heating and quenching) step.
In reality, any forge work on blades is limited not by available temperature, but on time of cooling against speed of hammering. In practice you can only shape over 4 - 6" in any given heat cycle before the metal is too cold to work. Remember that all historic sword smithing, reguardless of the culture or time, was done inside small diameter fires, by simply moving the metal through the fire as the work progressed.
You simply just do not need a long fire to forge knives. Even for medium blades or full length swords, you still only require a long fire for that single heat treating step *. If you forge a LOT of long blades, and do your own heat treatment, then it may be worth the time and materials to invest in a specialized forge for this process.

One other thing on the Lively illustration. It certainly appears that he is using charcoal BRIQUETS as the fuel. The lumps are the right size and shape - and far too regular in size to be natural hardwood charcoal. Although you CAN use briquettes, they are almost the worst choice I can imagine! The only thing they have going for them is wide availablity. Briquettes are formed from powdered clay soaked in used oil and charcoal dust. Thats why you get that heavy brownish ash from them, its the clay element. The heat will not be as even as natural charcoal, and there is a huge volume of ash and dust produced. The fuel lumps are also quite dense (heavier than properly coked coal). I suspect this leads to distortion of the metal as you try to thread it through the fuel mass. ( As you should be doing when heating metal, rather than simply laying the metal on top of the fire as I've seen so many beginners do!)

Darrell

* You also heat the forged blade to past critical (red) for the first annealing step. However as this step involves a long gentle cooling period, it is not as important to have the entire length at exactly the same temperature. I find that I can easily heat a blade that is up to three times the length of the width of my coal fire by simply moving it back and forth while heating. In my case that works for blades up to about 18 - 20 inches long.