Reticulation, an unpredictable organic-looking torch-created texture, is usually done with a silver alloy (also possible with a gold alloy). The technique is based on the fact that a pure metal and an alloy have different melting points and rates of expansion/contraction. A layer of pure metal is raised to the surface of the sheet of metal by depletion gilding (a repetitive process of heating and pickling to remove the copper oxide from the surface layer). The resulting sheet is then heated with the torch to a point at which the interior alloy begins to melt but the surface "skin" is only softened. As the piece cools and the alloy contracts, the surface becomes covered with ridges. Reticulation silver sheet can be purchased from jewelry suppliers like Rio Grande. It is not sterling. Its higher copper content lowers the melting point of the alloy, creating less risk of melting the surface layer and a more dramatic effect.
Another contribution credited to Seppä is his work with roll-printing. This is transferring a texture onto a metal surface using a rolling mill (not unlike what metal clay artists do to transfer a texture to the metal clay without the rolling mill).
A great book for both the anitclastic raising and reticulation techniques is Metals Technic, from Brynmorgen Press. Heikki Seppä contributed the chapter on Reticulation, and artist Michael Good, who studied with Seppä in 1979, contributed the Anticlastic Raising chapter. Good has developed and expanded the techniques in this form of metalsmithing and his work is simply stunning.
It's been ages since I've tried either of these techniques, but in the past few years I have thought often about trying my hand at the raising techniques again. I can make some time to do that, right?