Dr Moira Wilson selecting objects from Manchester Museum’s archaeology collection for rehydroxylation dating
Some time ago Sarah Jane Clelland who is Research Associate in the School of Mechanical, Aerospace and Civil Engineering at the University of Manchester, approached the Museum to seek permission to sample objects in the Museum’s collection to test a new dating technique called rehydroxylation. Sarah-Jane kindly agreed to contribute an explanation of the technique and how it works to the Ancient Worlds Blog. She and Dr Moira Wilson plan to test the technique using pieces of pottery found in the same pit as the Manchester wordsquare.
Ceramics, pottery or “pot” is arguably the most common find on archaeological sites worldwide. The predictable way in which fired clay material absorbs environmental moisture via a process called rehydroxylation (RHX) provides, for the first time, a method of directly dating archaeological ceramics. In many respects the concept of RHX dating is simple and can potentially provide a date of manufacture for archaeological ceramics. This would be very useful for archaeologists studying all periods of our past. In practise it is quite a challenge for the researchers to determine when the observed gain in mass of a piece of ceramic is due only to the rehydroxylation reaction. However, current research at the University of Manchester in collaboration with Quantachrome is working to address this. This research is led by Moira Wilson, who discovered that rehydroxylation proceeds at a predictable rate and co-invented the RHX dating technique. The idea that the gain in mass caused by absorption of moisture can be used to date archaeological and historic fired-clay by means of the rehydroxylation (RHX) technique is perhaps one of the most exciting developments in archaeological science since the introduction of radiocarbon dating in the 1950’s for organic material.
This is how it works: all fired clay – bricks, tiles, pottery – expands on aging due to the update of environmental moisture. The long-term moisture expansion of bricks has been known to structural engineers for some time, as it is the cause of cracking in brick masonry due to expansive stresses. This long term expansion is due to rehydroxylation, a super slow, progressive chemical recombination of environmental moisture with fired-clay material. Research in rehydroxylation has rapidly evolved from the prediction of expansion in structural masonry to the development of a potential dating method for archaeological ceramics. The process of Rehydroxylation (RHX) is defined as the chemisorption of atmospheric water by fired clay ceramic material which proceeds by ultraslow, nanoscale solid-state transport (single file diffusion) into the body of the clay. Ceramic material is very porous so water as vapour readily permeates it. As water molecules chemically combine with the ceramic it gains mass. However this long-term uptake of water is both very small and gets progressively slower. This means that the water demand for RHX is extremely modest.
A piece of 18th century Coalport bone china, as seen through a microscope, this shows how porous ceramic is, particularly when compared to the glaze © Dr. Robert Freer
This slow progressive chemical recombination of ceramics with environmental moisture due to RHX provides the basis of this archaeological dating technique. By measuring the rate of mass gain due to RHX and measuring the total mass gained due to RHX since manufacture (obtained by reheating the sample to 500°C), it provides an accurate self-calibrating method of archaeological ceramic dating.
The rate of RHX is described by a (time)1/4 power law, this means that it is a super-slow reaction. Two distinct stages are seen when ceramics, both in the freshly fired state and after reheating, react with environmental moisture. The brief first stage, which lasts only for a few hours, represents the sample cooling down from 500°C and coming into equilibrium with the ambient conditions. The second stage is where rehydroxylation is taking place and we believe that this process continues indefinitely.
The RHX dating method, first the mass of the dry sample is measured (green), the sample is then heated to 500°C until all the rehydroxlyated water is removed, this can take several days of heating. Once completely empty of RHX water, the mass gain of the sample is monitored until the mass gain is only due to the RHX reaction (red). Once the rate of the RHX reaction has been identified, we can work out how long it would take the sample to get back to its original dry mass (blue).
Ancient Worlds 