Sizing
Qualitative Tests
Various sizing materials are used, among which starch, rosin and gelatine are important. The following methods of test are prescribed for identifying these sizing components.
Starch Sizing - Drop on a test piece with a glass rod a weak solution of iodine in potassium iodide, approximately 0. 005 N Alternatively, treat a hot water extract of the paper with the iodine solution. The appearance of a distinct blue colour indicates the presence of starch, the deeper the colour the greater the quantity of starch.
NOTE - A faint colour shall not be taken as evidence of added starch, as in rag pulp It is very difficult to remove starch from the raw materials.
Rosin Sizing - Take a test piece of paper about 200 x 25 mm, pleate it repeatedly, place it in a test-tube and cover it with rectified spirit. Place the test-tube in a water-bath maintained at about 75 C till two-thirds of the rectified spirit has evaporated off. Remove the paper and evaporate the rectified spirit completely. All 1 ml of acetic anhydride in the table and dissolve the residue by warming. Cool it and add one drop of sulphuric acid of sp gr 1. 53. Formation of a fugitive violet colour indicates the presence of rosin sizing in the paper.
Gelatine Sizing - Cut a small quantity of paper from the specimen and boil for a few minutes in a beaker containing sufficient water to cover the paper. Pour off into a test-tube, cool, add a few drops of 2 percent solution of tannic acid. A flocculent precipitate indicates that the paper has been sized with gelatine. On heating the liquid, the precipitate will coagulate and cling to the sides of the test-tube.
Casein - Make a weak sodium carbonate or sodium borate extract of the paper, filter it off and add dilute acetic acid to the filtrate. Any casein present comes down as a white precipitate if this is filtered off and washed, it will give a purple coloration on warming with strong hydrochloric acid. If Millon's reagent is used with the neutralized extract, a red colour develops on warming. This reagent is prepared by dissolving 5 g of mercury in 10 ml of concentrated nitric acid and diluting with 50 ml of distilled water after the mercury has been completely dissolved in the acid.