Page:Cyclopedia of Painting-Armstrong, George D (1908).djvu/16

8 trace of nitric acid. An excess of sulphuric acid is then added to precipitate the whole of the lead and lime as sulphates, which appear as a white heavy powder. The beaker and its contents are then cooled by immersion in cold water. When cool, double the bulk of alcohol is added, and the whole allowed to stand for some time—over night if possible. It is then filtered and washed with alcohol until the washings are no longer acid, dried, ignited in a porcelain capsule, and weighed as above. From the weight obtained deduct .05 grain, the amount of ash contained in an ordinary Swedish filter paper, the remainder multiplied by five for the percentage of sulphate of lead, and (if chalk is present) this again by .852 for the percentage of white lead. If the sample contains chalk, the percentage of white lead must not be estimated until the percentage of chalk is determined. The percentage of chalk is converted into its equivalent of sulphate of lime. This latter deducted from the percentage of sulphates of lead and lime, and the remainder multiplied by .825 for the percentage of white lead.

Chalk. The readiest way to estimate chalk is to divide the nitric acid solution obtained above into two equal parts, in one estimate the lead as above (multiplying the weight found by ten instead of five), and in the other estimate the chalk. The lead is removed with ammoniac sulphide in the manner described before, to the clear filtrate, ammoniac oxalate is added, if chalk is present, a white precipitate of calcic oxalate is produced. This precipitate is assisted in its separation by boiling. Then collected on a filter, washed with hot water, dried and weighed. The ignition is complete when the contents of the capsule assume a tinge of gray color. The weight obtained after deducting .05 grain of filter ash is multiplied by ten for the percentage of chalk. In inexperienced hands it will be better to ignite strongly and weigh as lime, multiplying by ten for percentage of lime and by 1.7857 for percentage of chalk. This chalk may be