In the process of drying, contraction occurs. The various atoms of which the coatings are composed move closer and closer together; and as this contracting force is easier with than across the grain, cracks at right angles to it are formed. This fact suggests the necessity of so adjusting the elasticity of the various coats that the force exerted in drying may be as nearly equalized as possible, as their contracting force is continued until all elasticity has left the paint and oxygen ceases to be absorbed, all the oil acid has disappeared, and nothing but a hard, brittle surface remains.
Under the microscope, in the first stage of cracking, the surface presents nothing unusual except that the cracks appear clean cut and sharp on the edges. As months pass by and the surface is exposed to the atmospheric changes of heat and cold, wet and dry, the cracks become more numerous; and in the last stage, when the oil is entirely destroyed, the surface assumes the appearance of innumerable rectangular masses, higher in the center than at the edges, like small mounds raised by the process of contraction and adhesion.
Cracking in color coats may, by careful attention to preliminaries, be reduced to a minimum, provided good first-class materials are used and sufficient time is given to each coat to dry.
Where varnish is to be applied as a finish, all coatings should have oil in their composition and yet be mixed to dry flat. They should be applied very evenly and thinly, even if it necessitates an extra coat, to cover and make a solid job.
Striping and ornamenting should be done on flat color, which gives time for hardening, and fits it for the varnish coats to follow. If work is done in this way, I think very little fear of premature cracking need be entertained; at least, not until time and weather have sufficient opportunity to play havoc with its beauty, and natural decay of the materials themselves necessitates a thorough overhauling and repairing.
Rubbing varnishes are another source of trouble, causing the succeeding coats of finishing varnish to show signs of cracking long before they otherwise would, as it does not agree with the slower drying varnishes usually applied above it, being of a harder and more brittle character, serving the purpose of producing a fine, smooth surface, but sacrificing the durability of the job.
Concerning the cracking of varnish, I have not much to say. It seems to me that many of the reasons given above will apply to it as well as to the paint.
Poor material in the shape of varnish is poor indeed. A first-class article only will give first-class results.
It must be elastic, or it will crack easily and badly, no matter how good the undercoats of paint may be.
Good varnish on good color coats will not give any signs of cracking until, by repeated varnishings, it has accumulated a thick coating of brittle, unelastic gum.