Bead Furnace Experiments: Clay Mixtures

In May, Bruni, assisted by Aibilin and I, built several small open clay bead furnaces. We were attempting to make something similar to the less period firebrick furnaces we have been working with, but this time using a more period appropriate building material for our furnace.

The excavations at Ribe found flat hearths made of clay. However, the primary source about Ribe we read (cited below) did not give us much detail about the type of clay used. However, the article we read did note that refractory clay was found mixed in with the clay at at least one of hearths at Ribe.

Because not much information was available from the Ribe excavation about what material the bead furnaces were made from, the purpose of this experiment was primarily to test a few different combinations of building materials.

Two of the small bead fireplaces we made. Labels noting what materials we made the fireplace are visible carved into the clay in the picture.

We made small open fireplaces out of two different types of clay, ball clay (recomended by the owner of the ceramics store for this project) and stoneware clay, a clay that Bruni was familiar with from her work with ceramics.  We also tried mixing the ball and stoneware clay each with the refractory clay that was mentioned in the article about Ribe. Refractory (or Fire) clay is a type of clay that is more heat resistant than other clays. It is what the firebrick we used in our earlier experiments was made from. Finally, we also added sand to the mixture. Where we mixed more than one type of material into the clay used a simple 1 to 1 ratio of materials.

Note: we may want to try earthenware clay in the future
http://pottery.about.com/od/understandclays/tp/claytypes.htm

The addition of sand was recommended by the owner of the clay shop where we bought our materials. We had also seen sand being added to clay in other period and post period furnace/oven constructions. 

The sand is useful and important for a particular reason. When we were reading more about ceramics we found the following quote about primitive ceramics:  "objects made of a coarse, open body with plenty of large particles in the clay will tolerate more variations of thickness and fire with less risk" than an object made from finer clay particles and a thinner more uniform walls (Philip Rawson, Ceramics, 25). Later on, Rawson goes on to discuss more "primitive ceramics" which are put in direct contact with open fire, and which tend to be coarse/open bodied with big particles as a result (47). As our bead furnace would not be able to be fired in a kiln, we needed to find a combinations of building materials that could stand being exposed to direct fire.  As Bruni, who has taken many ceramics classes at U Penn, noted, when the water in clay is heated quickly as the kiln is being used, it will turn into water vapor. That vapor expands, and if it is trapped in the clay, it and can cause the furnace to explode when it is used. If the furnace is dried fully before use , this will be less likely to happen. However,  as the clay furnace dries, it will also shrink as it looses water, which can cause the furnace to crack even before it is used. The sand helps prevent the furnace from shrinking as much as the clay looses water and dries. It also allows the water an easier path of escape as it heats if it is not fully dry before use.

We built the clay fireplaces one weekend, and let them dry until the next weekend we all had free to play with them (about a month later). The goal was to stress test them, to fire them quickly and see which one either did not break, or lasted the longest.  We could then use this mixture to build bigger volcano shaped bead furnaces, or beehive furnaces. We did not time precisely how long each fireplace lasted, but we were able to make general observations about how long each fireplace worked before it started cracking. The small fireplace made of just ball clay cracked before  reaching a temperature sufficient to melt the 120 COE glass and we couldn't even start to make beads with it. The one made of only stoneware clay lasted a bit longer after we started a fire in it. When refractory clay was added to the mix, the fireplaces seemed to resist better. They seemed more able to withstand the heat, and we could use them for a time, but there was still some major cracking. 

Example of one of our clay fireplaces that cracked.

Overall the fireplaces with the sand and refractory clay both mixed in with the ball or stoneware clay worked the best by far; there was only minimal cracking in those fireplaces and we were able to use them to make beads using the softer 120 COE glass. We had to stop our experiments before those fireplaces failed due to incoming rain."Clearly the sand was important to the mixture for the reasons stated above.

Using one of the fireplaces to make beads from a glass rod.

Using one of the fireplaces to make beads by winding the class from a punty.

Source: Bencard, Mogens. (n.d.) Viking Age Crafts in Ribe: A summary.   Translated by MichaelaGibbion. Retrieved from: http://www.darkcompany.ca/articles/glasperlen.php