Corrugated rubber coating
A rubber corrugation is created by breaking the rubber shortly after the rubber coating. This breaking is carried out by special rollers . It should counteract the unwanted curling of the paper . As the gum dries, the stamp paper remains smooth.
Rubber corrugated stamps have very narrow wavy lines lying close together on the gum. A corrugated rubber coating can sometimes be recognized even before the reverse side of the stamp has been examined; it can vary in strength; if the rubber ribbing is particularly strong, it shines through onto the front of the stamp. In this case, the corrugation can also be seen on stamped stamps (gum was washed off).
The philatelist differentiates between two different types of fluting. The rubber corrugation can be horizontal or vertical to the stamp. This is called "horizontal or vertical rubber corrugation". Some stamp issues have both variants.
Rubber corrugations are very popular with collectors, especially when there are different variants for the same value (for example, vertical, horizontal and without). Rare types of corrugation often fetch high prices among philatelists. However, they are generally placed face-to-face in the collection.
Example of corrugation: Second costume series from the Austrian Post.