Monday, August 24, 2015

Period Bead Making at Pennsic

At Pennsic this year I helped Bruni make a clay bead furnace in the volcano style (See Dudly Giberson's "Volcano Dream" article)  during peace we. Together with Aibhilin we fired it and mad a few beads the Sunday of war week.

Constructing the Furnace
In this experiment we were building off of our previous experiment where we determined that a mixture of sand, refractory clay, and regular clay worked best. However, we also tried a few things differently this time:

1. We used grog instead of refractory clay. Reading volumes 4 & 5 of the actual Ribe excavation reports  (not the summary by Mogens Bencard published on the DARC company's website which we used for our last experiment) I did not see the word refractory clay used. Grog, however, was mentioned in the actual report from Ribe as being found in some of the bead workshop hearths (Sode 2004). Grog is basically ground up/crushed pieces of already fired clay  (Piepenbrg 432). Because these ground up pieces of grog have been pre-shrunk by firing, it helps prevent warping and cracking of the furnace just as sand does (Piepenburg 20).

2. We increased the amount of sand used. Bruni found a video about 18th century bread ovens where the builder used a high proportion of sand in their mixture. The author of the video noted that more sand than clay would help reduce the shrinkage and cracking in the oven (4:00).  

Note: this video also described using hay to add strength to the oven. This is something we plan to look into doing in a future experiment. Hay, being an organic material would likely not have been preserved at the Ribe workshop if it was used there.   

While it may seem strange to use a video of an 18th century bread oven as a source for a Viking bead furnace, the basic principles involved in building a furnace/oven seem to be similar across a wide time period. The author of this video noted that most ancient cultures have some variant of this type of oven, and that there are pictures of this type of oven from the Middle Ages. As I noted in the write up from our last experiment, the excavations at Ribe don't give us much detail about what made up the bead furnaces (just that clay and grog was used), so we need to experiment  to see what proportions work best in our mixture.  Modern archaeologists do something similar when they look to people today that make beads using a more traditional clay furnace (Sode 2003).

We built the furnace using 2 parts sand, 1 part grog, and 1 part stoneware clay. Due to time constraints, we were not able to take the time to build it quite as high as we would have liked, so future attempts will likely be a bit taller. We added in notches in the side for the mandrel to rest on as in Volcano Dream article. We let it rest for 3 days to dry before firing.

Firing the Furnace
Working in the furnace.
Once we managed to get the fire to catch (which took a bit), the furnace quickly got up to a temperature where we were able to make two beads (using the 120 coe glass).

We also took a video of us making a bead with our furnace!

By the second attempt at making a bead, the coals had gotten very low in the furnace, and we would have needed to add more and wait a bit for them to get up to temperature, to continue. If we had added more coals, we likely could have made more beads. Unfortunately, it was close to 11 by this time, and we had to stop so we could let the furnace cool down and then go to bed!  Pennsic, due to scheduling constraints, may not be the best place for our experiments.

Bead made in the furnace.

Condition of the Furnace after Firing Some thin cracks did develop in the furnace by the end of our working session with it. However, we were pleased overall with how the furnace stood up. None of the cracks that developed seemed to go all the way through to the inside of the furnace, or were large enough to cause us to loose heat while working.

Note Since sand and grog function similarly, maybe we just want to use grog in the future? Sand was found at Ribe, but it was described only as being layered between workshop levels (possibly spread during the annual cleaning of the site) not as being mixed in with the workshop hearths themselves (Sode 2004). We have also just found a few modern books and online forums on pottery which state that at high temperatures the sand in a clay mixture will start to crack due to the "quartz inversion of silica" (Piepenburg 20).

Crack in the furnace after firing.

This is likely the last experiment we will conduct this year. At Pennsic I was able to talk to Tinker from the mid (who was one of the people I first saw doing these experiments) and and get the names of some interesting books, and other resources (including notes on her own experiments). Much reading will happen, and then we will have new experiments hopefully in the Spring.


--Bencard, Mogens. (n.d.) Viking Age Crafts in Ribe: A summary.   Translated by Michaela 
--Gibbion. Retrieved from:
--Giberson, D. (1995). The Volcano Dream. The Glass Art Society Journal. pp. 77-84.
--Piepensburg, R. (1996). The Spirit of Clay. Pebble Press

--Sode, (2003). Viking Age Glass Beads from Ribe, Denmark, In Light of the Ethnographic Research, In. Ian C. Glover, et. al. Ornaments from the Past, Bead Studies After Beck. London: Bead Study Trust
--Sode, Torben (2004). “Glass Bead Making Technology.” In Mogens Bencard, Aino Kann Rasmussen and Helge Brinch Madsen. Ribe Excavations 1970-76. vol 5. Jutland Archaeological Society.  p. 82-102.  Note, I also looked at vol. 4 from the Ribe Excavations. This volume covered specifically the research done on the stratography of the site (the layers of deposit that make up the site).