Gum has had a long relationship with printing on postage stamps. In the pre-1930 days, flat press printing meant that sheets of paper were fed into the press one by one and then the printed sheets were hung up to dry, gummed, and weighted at the corners so that as the gum contracted the stamp sheets they did not curl. With the faster and more efficient printing method of rotary press, stamps were printed on rolls of paper. Hand gumming was impossible as was weighting each sheet to prevent shrinkage. The solution was to apply the gum to the sheet with ridges, sort of like expansion lines on freshly poured concrete sidewalks, which allowed a small amount of contraction of the paper from the drying gum without curling.
Gum ridges were the solution used by the US Bureau of Engraving and Printing. Different countries had different answers to this problem. The German printing office applied their gum in a sprayed ridged pattern against the curl of the paper—a high tech and very effective solution, but one which makes pre-1960 German stamps very difficult to convincingly regum. The British Crown Agents printers didn’t bother doing anything about the curling problem, and this is why even today if you leave a set of mint George V from any colony on your stamp desk overnight you come back in the morning to a tightly curled mess. The problem in general became moot about 1960 when technological advances in printing allowed stamps to be printed on rolls of pregummed paper in which the shrinkage from the gum had already occurred and been accounted for. But as collectors go back and collect earlier stamps, it is important to be aware of how earlier technology affects the stamps they are acquiring.