It will be seen, therefore, that sheets which have been prepared from rich latex or from too deep a layer of comparatively dilute latex will have a surface film of dry protein matter. Moreover, these sheets will be slow in drying, and in all probability will have a surface gloss and a dark colour. Hence it is not difficult to understand that some brokers regarded the presence of the so-called "rust" as an indication of over-smoking. To show that this is not so, and further that the presence of rust has nothing whatever to do with smoke-curing, it may be stated that _the presence of this protein film may be seen on unsmoked sheets_ which have been prepared from rich latex, from too deep a layer of more dilute latex, or from some thick sheets which have been rolled only very lightly. In fact, the presence of the protein film was noted on unsmoked sheet in 1910, when it was seen to resemble a thin yellowish glaze which could be scraped off with a pen-knife. Later, sufficient of this substance was removed from some very thick air-dried sheets, or thin slabs, to fill a small test-tube. When the sheets were bent or twisted, the apparent surface of the rubber (_i.e._, the protein glaze) cracked in all directions. In the case of sheets prepared from less rich latex, the surface film naturally is extremely thin, and no cracking is observed. If the fresh sheets are placed in a smoke-house, the drying film will take up colour from the constituents of the smoke, and it will be invisible. Somewhat analogous to the instance of a transparent glass giving a visible and opaque powder when crushed, so the transparent film on stretching breaks up into a visible powder which is lighter in colour than the rubber on which it is superimposed. It will be noted that since the introduction of standard methods of preparation, involving uniform dilution of latex, say, to a content of 1-1/4 or 1-1/2 lbs. dry rubber per gallon, complaints as to "rust" have decreased considerably. It is to be further noted as a peculiar fact that while two estates may be apparently working on identical lines, both as regards manipulation of latex and subsequent treatment of the coagulum, the rubber of the one may always be free from rust, while that of the other is often, if not always, condemned for the alleged defect. Obviously, in such a case, there must be an initial difference between the two latices as regards the percentage of proteins present; or there must be some small unrecognised difference at some stage of working. It will now be clear that "rust" is caused by a film of matter which is formed on the surface of the pressed coagulum, being there deposited by the exudations from within the rubber and through the pores. It is, therefore, necessary to avoid any conditions which will favour the formation of this deposit--_e.g._, allowing sheets to remain too long in a moist atmosphere before placing in the smoke-house.