acids may be regarded as a chemical combination of water and p 2 o 5 in a characteristic stoichometric ratio and the composition of the polyphosphoric acids may be similarly defined in terms of the weight percent of p 2 o 5 equivalent to said mixture based on total phosphorus content of said mixture determined by analysis termed the 34 p 2 o 5 equivalent 34 or in terms of the related weight percent of h 3 po 4 equivalent to said mixture termed the 34 h 3 po 4 equivalent 34 the term 34 h 3 ph 4 equivalent 34 is used in the following the polyphosphoric acid used should preferably have a h 3 po 4 equivalent of at least 100 particularly advantageously the polyphosphoric acid strength is between 105 and 125 h 3 po 4 equivalent in another preferred practice of the present invention commercial polyphosphoric acids having h 3 po 4 equivalents of 113 to 118 are employed as conditioning agents the amount of polyphosphoric acid used is preferably 2 20 parts by weight and particularly advantageously 3 8 parts by weight based on 1 part by weight of the 100 pigment the polyphosphoric acid pasting is generally carried out at 20 u00b0 200 u00b0 c preferably.