Page:Cyclopedia of Painting-Armstrong, George D (1908).djvu/17

Rh converted into sulphate of lime by multiplying by 1.36, and the product deducted from the percentage of lead and lime obtained above, prior to calculating the percentage of white lead. The chalk precipitate should be tested at the blow-pipe for its characteristic brick-red flame. Some analysts treat the mixed precipitate of sulphate of lead and calcium with concentrated solution of ammoniac acetate, and weigh the insoluble as chalk. This method is unreliable. The oil must first be burnt off, and the ignition continued until no black carbonaceous matter remains. It must be ignited in a porcelain basin over an Argand burner, turned low at first and gradually raised. The ignition must be completed over a Bunsen. Much time is saved and the analysis rendered more accurate by spreading the mixed paint in a thin layer over the bottom of the basin, and when the ignition is nearly complete, by crushing the scaly crust with a glass rod, carefully remove the adhering pieces on the glass rod by means of a feather. The difference in weight before and after ignition represents the oil plus loss by reductions of white lead to metallic lead. The residue is washed into a beaker with water as before, and afterwards with nitric acid. The metallic lead, which adheres strongly to the bottom of the basin, must be rubbed vigorously with the end of a glass rod until entirely removed. This requires a little patience, but it yields to persistent rubbing. The remainder of the process is the same as in the case of dry white lead.