In solutions of high gold content it is advisable to run a check with every batch, using a quantity that will compare with the expected value of the routine sample and following the same procedure as that of the sample assay. Check solution for this purpose is made up by dissolving exactly 100 mg of proof gold in aqua regia and making up to 1000 mL with water. To indicate differences in operation between pulp and solution assaying, the frequently used cuprous chloride method must be followed.
In high value samples, a 10 assay-ton (A.T.) aliquot is run into a 1000 mL boiling flask, 10 mL of Cu2Cl2 is added and the assay well mixed. Potassium ferrocyanide is added to coagulate the white precipitate and the contents of the flask then filtered. The filer paper with precipitate is opened and partially dried by being placed on a flat suction pad; solution flux is added and the assay put into a crucible for fusion and cupellation. The solution flux is made up with 34-38% sodium carbonate, 20-27% litharge, 22-28% silica, 10-15% borax and 1-2% maize meal.
For low value samples, specially shaped plastic containers are filled at the plant. On receiving these the assayer draws off the excess volume so that 2000 mL remains to be treated with cuprous chloride as describe above, but this time using a larger dosage of Cu2Cl2. These containers are then inverted over the filters to drain automatically, thus saving a considerable amount of manipulation.