If you are interested in gemstones and jewellery, you may notice some lovely red-orange stone beads with beautiful white designs on display in galleries 33 and 56 for instance. These beads are made from carnelian, a variety of quartz. The white designs on their surface gave them the common name of etched carnelian beads.
Etched carnelian beads were first produced in the Indus Valley around the middle of the third millennium BC. Early trade with Mesopotamia during this period probably sparked their local production. They then spread across the world, with archaeological examples found in various places, such as China, Iran, Iraq, Syria, Ukraine and Russia. Their shapes, sizes and the etched patterns are very diverse and their wide geographical distribution is striking evidence for the ubiquity of this technology.
To better understand the traditional process of etching carnelian beads, I tested recipes and processes reported in ethnographic studies from the 1930s in the Department of Scientific Research of the British Museum. These studies describe the etching agent as a ‘sticky paste’ composed of a washing soda solution and a plant juice. The plant documented as most commonly used is Capparis Aphylla, a bush growing in dry or arid areas in Africa, Iran, Pakistan and India. The etching paste is applied on the surface of the beads; these are then left to dry and finally fired to create the white design. As I found out while experimenting, the texture of the paste is crucial. It has to be fluid enough to be applied and draw complex patterns with relative ease, but viscous enough to adhere properly to the curved surfaces of the carnelian beads in order to prevent the designs from bleeding.
The first experimental challenge was to find a relevant substitute for the plant juice to obtain the right texture, since it was unavailable at the time of the experiment. Several chemicals were tested and the best results in terms of texture were obtained with sodium alginate, a seaweed-based gelling agent, commonly used in molecular gastronomy. I applied this optimised sodium alginate-washing soda etching paste onto unpolished samples of modern carnelian with a wooden toothpick and allowed them to dry. The second experimental challenge was to optimise the firing in terms of temperature and duration, as none of the written sources informed on this subject.
After drying, a transparent layer is visible on the surface of the carnelian sample. After firing, the etching paste has penetrated into the stone to create the white designs. The black residue on the white design is carbonised organic matter, which can be easily removed to fully reveal the design. The firing also enhanced the red colour of the carnelian.
A satisfactory result is however harder to obtain than one might think. If firing temperatures are too high or the duration too long, the carnelian will fracture or shatter. Conversely, if the temperature is too low though, the chemical reaction cannot take place.
Using digital microscopy and scanning electron microscopy (SEM), I recorded high-magnification images of both archaeological beads and experimentally etched modern carnelian. This allowed the physical study of the structure of the etched areas on a microscopic level, which revealed that the white colour may arise as a result of a network of holes, which scatter the light in a differently to the unmodified carnelian.
This scientific pilot study of etching carnelian, combining characterisation of ancient etched beads and experimental etching of modern carnelian, has yielded an insight into the highly- developed skills of the ancient craftsmen and the complexity of such ancient technologies. Further research is however needed to fully understand the mechanism and chemical reactions involved.
MSc Student in Chemistry at Chimie Paris-Tech and placement student in the Department of Scientific Research