Nature of leather -- Mineral tanning substances -- Salts of aluminium -- Alums -- Aluminium sulphate -- Effect of salt in tawing -- Basic alumina solutions -- Tawing of skins for rugs -- Calf-kid manufacture -- Glove-kid -- Green leather and other combination tannages PAGE 184 CHAPTER XV. _IRON AND CHROME TANNAGES._ Iron tannages -- Chrome tannages -- Chemistry of chromium compounds -- Knapp’s method of chrome tannage -- Cavallin -- Swan -- Heinzerling -- Hummel’s improvement -- Schultz’s method -- Theory of the two-bath process -- Practical management of the two-bath process -- Dennis’s chrome tanning liquor -- Procter’s liquors -- Theory of basic process -- Practical use of basic liquors -- Washing and neutralisation -- Effect of sulphur on chrome leather -- Bluebacking -- Fat-liquoring -- Dyeing of chrome leather -- Glazing and finishing PAGE 198 CHAPTER XVI. _PRINCIPLES OF THE VEGETABLE TANNING PROCESSES._ Methods of sole-leather tanning -- Finishing of sole-leather -- Theory of vegetable tannage -- Deliming of sole-leather -- “Mellowness” of liquors -- Penetration of tannage -- Drying of sole-leather -- Tanning of dressing leathers -- Preparation for tannage -- Avoidance of “bloom” -- Tannage of moroccos and other skins PAGE 220 CHAPTER XVII. _COMBINATION OF VEGETABLE AND MINERAL TANNAGES._ Early combination tannages -- Respective effect of mineral and vegetable tannages -- Use of fat-liquor -- Action of mineral and vegetable tanning materials on each other -- Danish and Swedish glove leathers -- Green leathers -- Making of fat-liquors -- Chrome combinations PAGE 236 CHAPTER XVIII. _VEGETABLE TANNING MATERIALS._ Distribution of tannin in plants -- Structure of barks -- Botanical list of important tanning materials PAGE 242 CHAPTER XIX. _THE CHEMISTRY OF THE TANNINS._ Sources of tannins -- General qualities of tannins -- Chemical constitution -- Catechol- and pyrogallol tannins -- Catechins -- Tendency of Catechol tannins to darken with light -- “Physiological” and “pathological” tannins -- Presence of mordant colouring matters PAGE 294 CHAPTER XX. _THE SAMPLING AND ANALYSIS OF TANNING MATERIALS._ The International Association of Leather Trades Chemists -- The American Official Association of Agricultural Chemists -- The sampling of material -- Preparation of solution for analysis -- Extraction of solid materials -- Total soluble matter -- Evaporations of solutions -- The weighing of residues -- The determination of non-tannins -- The hide-powder filter method -- The hide-powder shake method -- Determination of moisture -- Colour-measurement PAGE 300 CHAPTER XXI. _THE GRINDING OF TANNING MATERIALS._