Tuesday, September 08, 2015

Considering Ash in Bead Furnaces

This is a cross post from the DARC blog ...

One of the recurring problems the DARC bead making team has experienced is ash scars on the beads we make with various Viking Age type bead furnaces.

If readers are not familiar, you should spend a bit of time over on the main DARC web site - the area detailing our ongoing experimentation into Norse glass bead production :


It should be noted that Neil Peterson is the primary lead on this experimental archaeology project. (I only contribute as the 'fire guy', primarily on furnace design, with at least a basic level of skill with glass bead making itself)

As a (very!) brief overview :
• Plentiful archaeological remains at several Viking Age trade centre / town sites show that the Norse certainly made glass beads.
• 'Waste' remains do hint at some of the possible methods used to make individual beads.
• Raw glass itself does seem to be an imported material.
However :
Artifact remains are almost entirely limited to completed beads (graves) or waste products.
• There are only a very few actual glass bead making * tools * that have been recovered.
• There are no compete bead making furnaces. All that have been found are a very few clay 'bases' - that may (or may not!) suggest possible furnace dimensions.
All this, as practical workers, leaves us with a huge number of unanswered questions. (Summary of 'Questions & Answers' by Neil )

The experimental series has mainly concentrated on a couple of primary concerns:
• How do you build and operate a small charcoal fired furnace that allows you to effectively make beads similar to the ones known from the Viking Age?
• What is the superstructure for that kind of furnace - as defined by the remaining 'base plates'?

I have commented before concerning possible *effective* furnace design. (Admittedly, these based on my understanding of iron smelting furnaces and charcoal blacksmith forges.) :

Wednesday, October 10, 2012

"Bottle" Bead Furnace


The following is taken from an ongoing conversation Neil and I have been having about furnace design with an eye to reducing ash scarring. (As further background, Neil is proposing a series of tests that will produced measurable records of ash loading in the working area of the furnace.)

On 31/08/15 2:26 PM, Neil Peterson wrote:
... In passing on Sat you mentioned that chimney shape shouldn't impact ash.

Would you care to elaborate?  Feel free to suggest chimney ideas you think I should look into for this cycle.
You asked - much of this may be obvious to you

the ash produced ?
A given volume / mass of charcoal will produce a fixed amount of ash.
This will be a variable related I suspect most directly to wood type.
It may be effected by available air supply. (ie complete vs incomplete combustion)
It may be effected by actual temperature inside the furnace.

Wiki gives : "Typically between 0.43 and 1.82 percent of the mass of burned wood (dry basis ) results in ash " (this is talking about *wood* - not charcoal however). Likely this exact number can be researched (or close enough).
Given that in our case the wood type is not highly variable (usually oak or maple, standard sources).
The volume of the furnace is fixed (although changes between builds)
We are attempting to maximize temperature - kind of. Here we are balancing between duration and a fixed high temperature. To date we have been adjusting air volume to create what we consider an optimal temperature - of the exhaust gas (not necessarily the core burning temperatures, which I am not sure have been measured?)

So - we have an element to consider, which is particles over time.
1 kg charcoal yields 10 gm of ash (WAG) / complete burn time
Divide that by limited time that it takes to make a bead.

More important here is 'lofting' - how much ash is lifted into the expelled gases.
Generally we should have air input = gas output. (Scientifically there is some waggle here, based on relative volume of the elements in air against elements in the hot gas. I would think that not significant!)
Not *all* the ash produced is lofted. Much is trapped inside the remaining pile of charcoal inside the furnace.
This is especially true if there is a volume of 'non reacting' charcoal above the burning zone. This will be acting somewhat as a filter to trap particles.
Honestly, I think this is likely the major consideration. Bench test - compare ash visible in exhaust between start and end of a load cycle. Comparison test - how much ash is expelled from a smelting furnace (with about 40 cm of charcoal 'filter' above the combustion area)
Design implication? Build furnaces with a larger 'reaction area' - basically increase the bottom chamber height. (I did notice most of the builds others did on Saturday were very low furnaces - I expect these will have short effective working cycles, need more frequent loading, result in more ash in the exhaust.)

The other possible effect would be ash expelled from the reaction area, but because of height of the chimney below the working area for bead making, ash can settle back down into the chamber.
The core idea here is possibly increasing the chimney height.
I'm not so sure about this, as the ash particles are extremely light - and the air flow input is likely more than enough pressure / volume to still lift those a long way.
Increasing the distance or the complexity of the pathway of exhaust gas on its way out of the furnace may present a solution. The core problem here is doing this without loosing so much heat that the exhaust temperature drops below our needs.

The simplest way to construct a 'long chimney' would be with a tall, cylindrical furnace.
Primary problem is heat loss. There is the surface area (radiation loss) over internal volume ratio problem.
You might also use an internal set of baffels. This would in effect create an upper chamber with the
baffels above a lower chamber which in effect becomes a fire box. The upper chamber would suffer less heat loss - especially since one surface would be directly above the burning charcoal.
Primary problem here is complexity of construction.

Standard Bellows - actually based on that for Blacksmith's Forge.

In either case, one other consideration might be the air source. I'm not sure we are getting the correct balance between force and volume. Again our historic use is via a blacksmith's (speculative) equipment. I have certainly noticed massive variations in force and regularity of stroke between users. I think this is significant in terms of ash movement. What you desire is a slow, extremely even blow of constant pressure. It may be that a bag as a kind of regulator might be indicated.

It also would be valuable to have measurements 'in line' for air volume and pressure.
I would also suggest that input air from mechanical sources be controlled via an electrical controller (light dimmer) or a sliding gate of some. Both with markings at least relative - and repeatable.

What I meant about  'chimney shape not mattering' was an oversimplification and  short form. I think the elements given above are much more significant. There are theoretical considerations on shape (round / square) that likely do not come into play on this scale.
To my understanding, in large scale buildings, this is more to do with construction abilities than effects.
There is the 'passive draw' effect - which is a function of height, and does not apply here (scale again).
There will be some 'settling effect' - ash falling down after a certain distance. I think this also relates to reduced temperature due to cooling effects of the stack walls. Certainly NOT what we want here.
There is a 'lofting' effect - basically a tall stack distributes the same ash over a wider fall area (so reducing the individual point by point impact). Again not our concern.

We are more concerned with heat concentration - maintaining  a specific temperature at the stack mouth. A much wider chimney opening would certainly reduce the amount of ash present at a specific point location inside that opening. It would also be spreading the available heat energy out - certainly not a desirable effect in this case.

Without guidance from archaeology, anything we do is so speculative anyway.

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February 15 - May 15, 2012 : Supported by a Crafts Projects - Creation and Development Grant

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