Written by Simon O’Connor, a volunteer at the Barnham excavations.
In the wet sieving area at Barnham you will see rows and rows of buckets with thick chocolaty mud in them. This is the vital disaggregation stage that is needed to allow the sediments to wash away leaving stones, teeth and bones behind in the sieves. To achieve disaggregation there is a magic ingredient that we add to the sediment and water mix. The secret ingredient is a 1/3 of a cup of biological detergent powder.
To understand what is happening here we needed to delve into a little soil science. The sediments in our case are a mixture of sands, silts and clays. Sand and silt grains will not stick together, but clay acts like cement gluing everything together. The property of clay particles that causes this is a negative charge to each particle. If we added clay particles to distilled water they would disperse easily as the like charges of the particles repel each other in the same way as like poles of a pair of magnets do. However ground water in this area contains positively charged ions of calcium and magnesium that attract the clay particles causing them to clump together in a process called flocculation. Flocculated clay particles are the glue that cements the sand and silt together.
Detergent powder contains a water softening ingredient called sodium polyphosphate. When this ingredient is added to water the polyphosphate disassociates from the sodium. The polyphosphate has lots of negative charges and wraps itself round calcium and magnesium ions neutralising their charge in a process known as sequestration. The sodium ions replace the calcium and magnesium ions. They differ from calcium and magnesium ions in that they have a single positive charge rather than a double charge of the hard water ions. They also attract an insulating layer of water molecules (know as a hydration shell). For these reasons sodium ions are much less effective at flocculating clay particles. So a bit of a stir in water is sufficient to disaggregate our sediments when we add the magic 1/3 of a cup of detergent powder.
This proves that a few suds are more than a match for the rock hard sun baked sods at Barnham!