While not very often in demand, it will so happen, that for a special purpose, such grindings are specified. In order to avoid waste by evaporation and splashing, the grinding must be done on mills of slow speed, the stones being encased. Drop black, Prussian blue and ochers are rather hard to grind in turps, while white lead, zinc, lithopone and the chrome yellows, umbers and sienna are usually soft. Unless a small portion of a lubricant, such as oil or varnish is used in addition to the turpentine, the loss is entirely too great and it is next to impossible to obtain a smooth paste. The proportion of pigment, oil and turps required are about as follows, allowing for evaporation:

White lead:

90 pigment,

2 refined oil,

8 turps.

Zinc white:

72

5 "

23 "

Lampblack:

25

10 "

65 "

Prussian blue:

50

10 "

40 "

French ocher:

64

6 "

30 "

Chrome yellow, M:

60

5 "

35 "

,, green, pure:

64

6 "

30 "

Bone black:

50

10 "

40 "

Umber and sienna:

45

10 "

45 "

Vandyke brown:

40

10 "

50 "

It any ot these batches are to produce 100 pounds finished material, the portions of turps here given must be increased in the mixing by 10 per cent at least.