This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
This is easily proved by pouring a solution from one bottle to another, when it will be seen to flow in a light, frothy-like stream, much less dense than a solution of the unbleached article. Further evidence is in the fact that polishers using it in high temperatures are commonly heard to say that they cannot get it to lie flat, a term as applicable and correct as any, perhaps, when carefully examined; for the heat, acting upon the chlorine, which has undoubtedly entered into combination with it in the process of bleaching, causes that gas to expand, so that the more polish he applies, the more gas he has to contend with, in impeding that cohesion and crystallization which he is endeavouring to bring about.
Polish, under its most favourable conditions, is a compound so liable to change by variations of temperature, humidity, pressure, etc, that makes its use very variable and uncertain. Lac in its dry state, and in a temperature higher than is ever required for polishing, is totally unaffected; but put into boiling water, it speedily becomes soft and plastic, and on being removed from the water resumes its original character of hardness quickly, from its inferior capacity for heat. Not so is it with spirits of wine, its menstruum; this has an extraordinary capacity for heat, insomuch as that it will volatilize in the ordinary state of the atmosphere, its briskness increasing with increase of temperature.
Now, although boiling water has no action on lac, other than to soften it for the time it is immersed in it, having no power to dissolve it of itself, still that substance is very differently affected when in combination with spirits of wine, its true solvent, the strong affinity for heat of the spirit entirely overcoming the feeble capacity for it in the lac; and so strong is the affinity of the spirit for the lac, that it separates its last portions from that substance, when fairly combined, with the greatest difficulty. Thus the necessity, in polishing, of a moderate degree of heat, to assist that produced by the friction of the rubber, in forcing out that clinging portion of spirit before solidity and brilliancy can be obtained.
The most favourable temperature for polishing appears to be 60-70° F. (16-21° C.); ascending above this, one portion of the spirit evaporates before a proper distribution of the lac can be brought about, while the other portion, which adheres so tenaciously to that substance, impedes its solidification. Descending below that degree, there is a tendency in the materials to chill, the more especially if the room in which the work is done be at all damp, the activity of the evaporation being checked by the absence of heat necessary for its conveyance. This is an evil more easily remedied than the former as in most cases all that is required is to light a fire, and by that means supply the deficiency. Not so convenient would it be in the height of summer, with the thermometer indicating 80° or 90° F., to remove the work to an ice-house; and being so removed, the remedy would be worse than the evil. But of all the injurious influences attending polishing, none is comparable to humidity.
If the atmosphere be saturated with moisture, as it not unfrequently is, when the clouds, or aqueous vapour, instead of being buoyed up in the sky, hang about the earth's surface, even though the thermometer stands at 70° F., as favourable a point as any, polishing becomes extremely difficult; the materials appear to be so completely neutralized, as to render them incapable of performing their office. Increased pressure and friction seem inadequate to supply or make up for the atmospheric derangement. The cause of this may perhaps be thus explained: - All liquids in becoming solids part with heat. Now this liquid, being compounded of spirit, not only has it become enfeebled, being spread on a surface, and thus exposed to a body for which it has the strongest affinity, but becomes so diluted by it, that it has lost in a great degree the power of evaporation or means of parting with heat, consequently assuming the solid form with difficulty.
Atmospheric pressure is undoubtedly the surest guide to the experienced polisher, showing him the power nature is employing for his advantage, or detriment; for, carefully observing the movements of the mercury, he will not fail to realize the fact, that as it ascends his labour will be considerably lightened, while, on the other hand, it will be greatly augmented by a corresponding depression - regard being paid, of course, to temperature.
It may be proper, however, to acknowledge that this theory rests on supposition. It is nevertheless a fact, that when the air is most suitable to ourselves - when it is bracing and buoyant - infusing as it were more life into us, it is also found to be more suitable for the performance of our work. It must not, however, be inferred, from these remarks, that polish will not work under the influence of these atmospheric changes, for it is found to do so in our climate, even under its extremest fluctuations; but what is meant is, that its effects are less under a low than under a high pressure, in a moist than in a dry atmosphere, and either in a low or high temperature, than in a medium one. The cause of this may be thus explained : - By pressure, the polish is condensed, the spirit flying off to find its natural level, and thus favouring the solidification of the exposed strata of resin. The dry atmosphere offers facilities for the escape of the spirit; whilst the moderate temperature so regulates its volatility that it neither passes off before the resin can be properly worked, nor remains inactive in discharging the necessary amount to produce solidification.
From observations of the effects of polish, together with its daily use, the following conclusions present themselves, namely, that it is not in the nature of the materials, as at present compounded, to withstand the antagonistic influences constantly opposed to them; that the effects produced on polish by variations of temperature, show the necessity of so preparing it as to render it proof against such changes; and, finally, that it be so prepared as to withstand a much higher degree of heat than in its present simple form it is able to do. (John Dalton.)