The best material for a strop is good calf-skin, well tanned, and firmly glued to a strip of wood with the hair side out. The leather should be hard, otherwise it will rapidly round the edge of the knife. The way in which this occurs is readily understood from figures 11 and 12. In Fig. 12 the leather is seen to rise up behind the edge of the blade as the latter passes over it, so that instead of two plane facets meeting at an angle of from 15° to 20° as left from the hone, (Fig. 12) the edge has become considerably rounded and the ultimate angle of the cutting edge is nearly or quite doubled. The hardest and firmest leather should therefore be chosen, avoiding of course any leather that has been made hard and unpliable by the action of water or other agents. Two sides will be found enough for a strop. On one the leather should be kept clean, while the other should be thoroughly impregnated with fine grained, but very hard rouge or crocus.[22] Rouge is an artificial oxide of iron prepared by exposing sulphate of iron to heat. The hardness of the resulting powder depends upon the temperature to which it has been exposed, and this temperature is very well indicated by the color which the rouge assumes. Bright red or crimson rouge is soft and will not cut steel; hard rouge, suitable for polishing steel, is purple in color, and this quality should therefore be chosen. Great care should be taken to see that it is free from gritty particles, and it should be well rubbed into the leather in a dry state. All mixtures of grease and oil with abrasive substances should be eschewed. If the leather be of good texture and the rouge hard and fine, a very few strokes will suffice to impart the last degree of smoothness and keenness to the blade. Footnote 22: Rouge may be purchased from dealers in watch-makers’ tools. Those who cannot procure it readily may very easily prepare it for themselves. Full directions may be found in the “Amateur’s Handbook.” New York: Industrial Publication Company. Since grit and dust would soon spoil the strop, it should be kept carefully covered and protected. The rouge will require occasional renewing, which may be done by sprinkling a little lightly over the surface and rubbing it in with the finger. Consequently we find that the best and most convenient holder for rouge is a bottle with some fine but porous fabric tied over the mouth of it, forming as it were a sort of dredge. Over the bottle mouth and its cover should be tied a paper cap to keep out dust.