Support Marks.—It frequently occurs that one sees across the middle of smoked sheets a wide mark. This is where the wooden support in the smoking-chamber has been. As a rule, even in the most careful cases a faint mark may always be seen, but in many instances this mark is exaggerated to such an extent as to point to lack of care on the part of the store supervision. If bays of racks remain empty over-night, they may possibly become covered with a light sprinkling of fine wood-ash and tarry deposit. Wet rubber placed upon these racks will pick up and retain the impurities, and more often than not they cannot be washed out. It is incumbent upon the manager to see that empty racks are thoroughly cleansed before placing wet rubber upon them. The better plan is to arrange that the bars can be removed easily from sockets. There should be in stock sufficient “spares” for, say, two days’ rubber. When the dry rubber is removed, the bars should likewise be taken away, to be cleansed and kept in the factory until again required. This will ensure that fresh rubber always rests upon a clean support.
On some estates, in order to guard against a pronounced “bar-mark,” sheets are moved and turned daily.
In other smoke-houses the upper surface of the bar is chiselled in concave form, so as to admit of the passage of smoke below the surface resting on the bar.
Stickiness.—This is not to be confounded with “tackiness,” from which the rubber does not recover. Stickiness is only temporary, and may be remedied. As a general rule, it is due to packing sheets, which have not a good raised “ribbing,” and which may have been coated with light tarry deposits (see [Glaze]). This surface film may be removed by washing the sheets, or scrubbing them, with cold water. Usually a further two days’ air-drying will make the rubber fit for packing; and if the smoke-curing has been efficient, there should be no need to anticipate trouble from mildew. Some estates adopt this practice daily with success, as a form of insurance against complaints of surface deposits.
Ribbing, Surface Pattern.—While we know that the passing of sheets of rubber between rolls, causing a particular raised pattern to appear, has no effect upon the actual quality of the rubber, there is a great deal of practical advantage gained.
The practice ensures an increase of superficial area which is an aid in drying, improves the appearance of the rubber for selling purposes, and is of distinct advantage in enabling the rubber when packed to travel in better condition. Sheets do not become so closely packed; sampling and general handling are easier on delivery.
As long as the plane surfaces are sufficiently and regularly distorted, there would seem to be no limits to the type of pattern or “mark” which may be placed upon the rubber. But in actual practice the variety is small. The most popular type of “ribbing” is that best described as a small diamond effect, produced by a pair of rolls cut with closely placed narrow grooves running spirally. The spirals travel in the same direction on both rolls, producing close-cut ribbing running in opposite directions on the surfaces of the sheet. On sheets of standard thickness, the result approaches a diamond effect.
A few other patterns are employed, notably that producing longitudinal stripes of varying thickness. On the whole, the type of pattern would seem to be immaterial, if the points already indicated are achieved.
It is seldom one encounters a case nowadays in which the “marking” is unsuitable, but a few estates may be using an old type of patterned roll on which the full diamond grooving is cut. As this appears on both sides of the sheet of rubber, and as the ribbing does not coincide, a blurred effect is seen when the sheet is viewed against the light.
Thick Ends, “Sheet Clippings.“—It rarely happens, even with good equipment and average supervision, that the preparation of smoked sheet is unaccompanied by slight defects. For instance, in spite of rules and regulations regarding manipulation of the coagulum, it is not uncommon to find that some sheets, after rolling, have slightly thickened ends. In the ordinary course of events these might delay drying considerably. It is the practice on some estates to cut off these thickened ends while the rubber is still wet. The pieces are then machined into crepe form, but as no sodium bisulphite may have been used, the resulting rubber cannot be classed as No. 1 Standard Crepe.