Bead Furnace Burn July 20 2008
: Cobb (Straw, clay, sand), charcoal, glass
: Electricity was available, a mix of first time beaders and experienced people.
This day of beadmaking was focused on
- the creation of the next generation furnace
- validation of the internal airflow
- lampworking in the chimney
- working with tesserae
The Mark3 furnace retained the base plate size of the Ribe oval (20x60cm), but had several modifications:
- Internal diverter for the air
- two chimneys
- annealing chamber
- working ports moved to ends
- move access ports to the side with the air entry
The air diverter was a V shaped piece of clay with the point facing the air opening. The sides were slightly curved to send the air into the ends of the furnace. This was intended to shift the first from having a single hot zone in the middle to having two hot zones, hone of each end of the furnace. Although this piece has no analog in the archaeological record there are a number of ways it could have been done that would not leave a trace. In this instance we are not trying to prove that this was used, just exploring some options.
The two chimneys were built slightly differently to test which style worked better.
The annealing chamber was added to leave the chimneys' free to improve the draw. It was filled with ash as an insulator.
As with the Mark2 we had people making beads using standard lampworking techniques. In this case we made them in the working ports, and in the chimneys. This photo is a bead being worked in the low chimney. Working within the chimney was quite possible but if the working port was not well sealed the heat rising from it made the dynamic of using the chimney difficult.
New in this melt was the use of tesserae. These are small pieces of glass used in the creation of mosaics. They are found on a number of sites in the viking era and are believed to be the source of glass used in the manufacture of beads.
To simplify the experiments by removing some variables the tesserae procured were broken pieces of fusing glass with a matched CoE of 92. By keeping the lampworking glass (CoE 104) separated from the tesserae (CoE 92) we hoped to reduce the issues around annealing the glass and CoE related breakage. When working with glass it is important to note that it expands when heated and contracts as it is cooled. If you make a bead with two pieces of glass with different CoE's then as the bead cools the pieces will pull apart and break the bead. While it is unlikely the norse had the luxury of matched CoE glass it should be possible to overcome this problem with annealing (slowly cooling the beads). Using a matched CoE glass simply sidesteps a difficult and frustrating problem until the basic skills have been mastered. We lose enough beads to breakage now due to other issues we don't need to make it worse with a known problem!
Working with tesserae adds a level of complexity to making the beads. The tesserae are odd shapes which means that unlike the stringers & rods, it is difficult to evenly apply the glass to the mandrel. There is also the difficulty of determining how to heat the tesserae so that it can be wound on. We used a number of methods with varying levels of success. Unfortunately in attempting to develop a new skill it is difficult to know if early problems are because the technique is not workable, or if it is just not yet mastered.
We used the following techniques:
- hold the tesserae in tongs until heated (not particularly good)
- Placing the tesserae on a piece of charcoal
- Placing the tesserae on a piece of soapstone
- placing the tesserae on a metal plate (like the one at Ribe)
It is also possible that the tesserae were melted in crucibles and pulled into rods and stringers.
With the exception of holding the tesserae in tongs all of the method work to one degree or another and all have some issues.
This photo shows the remains of a tesserae on a piece of charcoal and the bead being made from it.
Working over the chimney holding the tesserae in tongs. This technique was rather difficult due to the radiant heat.
Moving the bead from the mandrel through the annealing process is another area of the bead making process that produced unexpected difficulties. In this photo are four beads on mandrels, a couple of broken beads in the ash, and an unbroken (white) bead in the ash. Removing the bead from the mandrel has produced some breakage. Our current guesses as to cause are
- Imprefections in the mandrel
- if the mandrel is bent even a small amount then the bead will break. The clay slip should allow for minor problems as it would wear away making a larger opening but this would only help with the smallest problems. We have been using stainless steel welding rod, the two archaeological samples are both tapered rods.
- Imprefections in the slip
- the mandrels are coated in a clay slip. When this slip cracks or flakes off exposing the metal this produces a possible problem. Glass in contact with the metal will lock the bead making it impossible to remove without breaking.
- Weakness in the glass
- Heating and cooling the glass rapidly puts flaws in the crystal structure. This is usually mitagated with annealing. Trying to remove the bead before it has been annealed can be higher risk if the bead has more than a normal number of flaws.
- Shape flaws
- When the glass edge is not even and solid across the leading edge of the bead it can produce focussed stresses in the glass as it is pulled off. In the picture above the green bead on the right has an uneven edge. As we pull the bead up and off the mandel there will be different levels of friction/stress on the upper left and lower right portions of that bead, increasing its risk of breaking.
This picture (above) also shows two methods of annealing being attempted. Leaving the bead on the mandrel and leaving it near the fire to cool, or removing it and placing it in the hot ash.
The photo also shows traces of ash on some of the beads, this gets attached while the bead is being worked and would mar the bead surface permanently.
This image shows the materials used in beadmaking. In the lower right aboce the tweezers are two red tesserae. The tweezers themselves are useful for placing tesserae into the furnace. In this case they are modern stainless steel tools while a forged set is being made. A clear blue glass rod, and a white stringer (thinner rod) are visible in the center with green and light blue rods on the left. Additionally there are bits of glass broken from the rods, and some beads which were broken longitudinally. A spare manrel with wooden handle is visible at the bottom along with a set of forged tongs.