CYCLOPEDIA OF PAINTING.
ADULTERATION OF PAINT.
To understand this subject intelligently it will be necessary to possess some slight knowledge of chemistry and of the materials used by chemists, but any intelligent person can, by careful reading of these descriptions, test substances qualitatively without the aid of expensive apparatus or external assistance of any kind. For quantitative analysis a delicate chemical balance will, of course, be required. Take paints first. White lead is now very seldom sold in the dry state, but samples are occasionally met with. Its covering power being superior to that of any other known paint, it is very largely used, and it is frequently adulterated with substances of inferior quality. The most common adulterants are sulphate of baryta and chalk.
Sulphate of Baryta. Treat a small quantity with dilute nitric acid, and heat on the sandbath. If any insoluble remains, it is either sulphate of baryta or insoluble silicates. Filter, take a portion of the insoluble on a piece of clean platinum wire moistened with hydrochloric acid, and test at the blowpipe. If the flame be colored green, the precipitate is sulphate of baryta. By moistening the wire in hydrochloric acid the green color is reproduced many times.
Insoluble Silicates. If no green color appears, the insoluble is a silicate. This may be proved by forming a bead with microcosmic salt on a loop at the end of the platinum wire, and taking some of the precipitate on this bead, fusing it again in the blowpipe flame. If small infusible particles whirl around within the bead while in the flame, the presence of silicates in the precipitate may be inferred.
Chalk. The presence of chalk can only be ascertained after separating the lead. This is best done by adding ammonia solution to the nitric acid solution until alkaline, then sulphate of ammonia in excess, and boiling for five minutes. Filter off the black precipitate which is formed, and to the filtrate, first tested with an additional drop or two of sulphate of ammonia to insure the removal of the whole of the lead, add ammoniac oxalate. If a white precipitate appear, it is calcic oxalate. Test a portion of this precipitate at the blowpipe. A brick-red color imparted to the flame verifies the presence of chalk.
White Lead. The presence of lead should be ascertained. This is indicated by the black precipitate given with sulphate of ammonia. It may be best ascertained by boiling the nitric acid solution to expel the free nitric acid, adding dilute sulphuric acid to the clear solution, dissolving the white precipitate of sulphate of lead thus formed in ammoniac acetate, and adding potassic chromate to this solution. A heavy yellow precipitate (chromate of lead) forms when lead is present.
Sulphate of Baryta and Silicates. Take 20 grains of the sample, treat with dilute nitric acid as before. The quantity taken for analysis may be weighed in a watch-glass or a small basin, but should be transferred to a beaker, and the glass or basin washed with distilled water before the acid is added. If this precaution be not taken, and the acid added directly in the watch-glass or basin, to be washed into the beaker afterwards, the violent effervescence which takes place on the addition of the acid will occasion considerable loss by spurting. If, after heating with nitric acid, an insoluble remains, a few crystals of chlorate of potash may be added to the boiling liquid to insure the solution of all soluble substances. The boiling is continued for a few minutes, then cold water is added, and the whole passed through a filter. The insoluble on the filter is washed with hot water until the water leaving the filter is no longer acid to litmus paper. It is then dried on the water-bath, ignited, and weighed. Test at blowpipe as before. Sulphate of baryta and silicates, if both present, are not usually separated.
Weighing Precipitates. This general direction applies to almost all precipitates. If strong acids or acid and chlorate of potash are used, the liquid should invariably be diluted before filtering. In washing, allow the whole of the wash water to drop from the filter before adding more water. When thoroughly washed, fold the filter paper flat in the funnel, or, better, transfer the filter paper and its contents to a large watch-glass or flat basin, and dry on a water-bath. When dry, carefully unfold the filter over a quarto sheet of stiff glazed paper, with a feather brush off every particle of the precipitate adhering to the filter paper, collect the precipitate on the glazed paper, again using the feather, and cover over the precipitate with the funnel. Fold the filter paper till it assumes the appearance of a solid cylinder about one inch in length, and ignite with the Bunsen burner over a weighed platinum or porcelain capsule. After a while the filter paper becomes a charred mass of smaller dimensions, and drops from the wire into the capsule. The wire is cleaned into the capsule, by means of the feather, of any adhering particles, the charred paper is crushed with a glass rod, and ignited over a Bunsen until the ash is no longer black. The capsule is then stood on a porcelain slab, and the precipitate carefully transferred from a glazed paper into the capsule, the feather being employed to remove the last traces. The capsule is then ignited over an Argand burner until the weight is constant. The filter paper must always be ignited before the bulk of the precipitate is transferred to the capsule.
White Lead. The nitric acid solution obtained as before is boiled nearly to dryness, and if a precipitate forms a little water is added, then dilute sulphuric acid in small quantity, and the boiling continued for some minutes to expel the last 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 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.