Washing away the dirt: the use of hydrogen peroxide on soil samples during environmental processing

Environmental remains, which include material such as animal bones, charred plant remains, charcoal, pollen, molluscs and insects, have the potential to give us key information about the sites we excavate. These remains provide vital information about things such as diet in the past, what a settlement’s economy may have been based on, what the local landscape surrounding a site may have looked like, and how this may have changed over time. While some of these pieces of evidence are often found by our archaeologists working in the field, particularly larger animal bones, remains such as burnt seeds, charcoal, snail shells and small animal bones, are so small that they can be easily missed during excavation.        

Our archaeologists therefore regularly take buckets of soil samples from features like pits, hearths, ovens and ditches, so that we can recover small environmental remains that would otherwise be lost. In order to find the material within the soil samples, the soil needs to be washed away so that we can get at the precious bits of evidence potentially hidden inside. However, it can be hard to get samples from sites with clay soils cleanly processed without damaging the delicate environmental remains.

One thing that helps is presoaking the samples with a mixture of water and hydrogen peroxide. The hydrogen peroxide works a treat, fizzing away and breaking down the bonding between the clay particles, while it converts from H2O2 to water and oxygen – the reaction gently loosens the bonding between the clay particles, so they reduce in size until they are small enough to go through the mesh on our wetsieves. We then use a flotation tank to wash the clayey sediment away, allowing us to float off the light environmental remains, with a residue of stones and small objects left over from the soil.

Without using hydrogen peroxide the process takes much longer: the clay samples can become plastic-like as the clay particles keep rebonding, and it is often hard to get them through the sieve without a lot of agitating of the sample by the processor – this agitation can damage the environmental material, leading to a loss of the important information it can contain.   

Sarah Wyles