Phosphate Tests for the Amateur Archaeologist

tcablogmaster has long wondered  if more information could be gained from the intriguing sequence of layers revealed by our excavations at Millhams.  These layers represent the deposition of silt and sand following regular inundation of this marshy site by the adjacent river Avon, as revealed by the inclusion of tiny water-living mollusc shells in the layers.  One analysis technique used to determine the presence of human occupation is phosphate analysis, since phosphate levels are high in human and animal waste, the inevitable accompaniment to human settlement.  Since we are looking for a suspected house or dwelling on the site, this seemed an appropriate technique to try in order to see if variations in phosphate concentration as a function of layer depth or location could be deduced.

The first experiment involved a garden soil test kit, using a makeshift colorimeter app in a mobile phone to try to determine the comparative concentration of phosphate in a sample of water used to extract phosphate from 10ml sample of soil.  Unfortunately the colour changes produced seemed to be very difficult to compare with the supplied colour chart and this method was discarded.

A second attempt at a qualitative test was an implementation of the so-called Eidt Ring Test using instructions on the SASSA website (Soil Analysis Support System for Archaeology) at http://www.sassa.org.uk/index.php/Analytical_Methods:Phosphate_Qualitative .  All the chemicals required – hydrochloric acid, ammonium molybdate and ascorbic acid – were available on ebay and using the recipe, tcablogmaster concocted Reagent A (ammonium molybdate dissolved in dilute hydrochloric acid) and Reagent B (a solution of ascorbic acid in water).  Deionized water readily available from motorists centres was used to make up the solutions.

PhospateTest_wordpressApplication of the test requires a tiny sample of soil (about 50mg) to be placed in the centre of a 90cm filter paper and 2 drops of Reagent A added.  After 30 seconds, 1 drop of Reagent B is added.   The development of the blue colour is then observed and the resulting pattern scored according to various, rather imprecise criteria, such as length of time for the colour to appear, its intensity and the radius of the ring.  A number of tests were conducted on the 4 layers identified in Pit 52 – the topsoil plus 3 layers of silt or silty-sand – and the results are shown opposite.  The topmost sequence shows the result of an onsite test – unreliable in many ways because it seemed that probably too much soil was used in the samples and also the filter papers kept blowing away!  Subsequent test were done at home in the greenhouse with the bottom row probably the most representative since care was taken to measure the weight of the soil sample.

From these few results and in the absence of experience with the technique it is difficult to form any opinion of the relative phosphate levels.  It looks as though the topsoil is the richest in phosphate it is difficult to say whether the difference is significant.  Clearly the test is comparative but there is great difficulty of judging and recording the intensity of the reaction for the purposes of comparison –  tcablogmaster would very interested in comments from anyone with greater experience of the technique!.

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