RUST-STAINS.--Rust-stains, on the other hand, throw the responsibility entirely upon the labour and supervision of the factory. Rust is formed upon the rolls when they are at rest, and any one passing pale rubber between the rolls before they have been thoroughly cleaned is guilty of culpable negligence. Even when apparently clean, a piece of lower grade rubber should be passed through the rolls several times so as to remove any slight trace of rust remaining. Rust-stains have also been caused in a few cases by the large knives which are used to cut up lumps of coagulum, or by allowing freshly coagulated rubber to come into contact with iron vessels in the factory. A similar appearance has been traced in a few instances to contamination of the coagulum in transit by the dust of the reddish rock (laterite) employed in localities for road-making. OIL-MARKS.--The origin of oil-marks in crepe has already been described in Chapter XIII. The whole question resolves itself into one of cleanliness, moderation in lubrication, and supervision. The machines should be inspected every day, and once a week rolls may be swabbed down with a 10 per cent. solution of caustic soda applied by means of a piece of cloth fastened round the end of a stick. Immediately after this operation water should be turned on and the rolls set in motion, so that all traces of caustic soda are thoroughly removed. If possible, lubrication by oil should be substituted by grease lubrication through screw caps. Particular attention should be paid to the back of the machines. None but the individual in charge of engines should be allowed to lubricate the machines, and he should be held responsible for any excess of lubricant. As a rule oil-marks are restricted to the edges or the proximity of the edges of crepe, but sometimes the streak is to be found in the middle of the length. In such a case it is almost certain that the oil or grease has been picked up by the rubber in the tray. It sometimes happens, if the "liners" of the bearings are eccentrically worn, that a few drops of dirty oil or a particle of grease are squirted out to some distance. These usually find a resting-place in the tray, and the contamination may then appear in any part of the rubber. It will be clear, therefore, that all trays beneath machines should be examined as the probable source of danger from contamination by oil and dirt. If the trays are as wide as or wider than the effective portion of the rolls, they should be discarded. In their place (except sometimes in the case of the macerating machine) all that is necessary is a movable piece of board, in width not less than from 4 to 6 inches shorter than the width of the rolls. Any oil or grease ejected from the bearings will thus be allowed to fall clear of the board; and defects due to oil streaks, etc., will be very much diminished, if not entirely obviated.