I’d never heard of this till I went to the Images of Value: The Artwork Behind US Security Engraving 1830s – 1980s exhibition at The Grolier Club. Siderography is a steel-based transfer method used in the engraving of bank-notes.
The U. S. Bureau of Engraving and Printing (unsurprisingly) has a definition of siderography and they also reveal to us the existence of the exciting word siderographer. The Oxford English Dictionary, trusty guide, informs us that sidero- is a prefix meaning “relating to or containing iron”.*
One of their quotes, from 1900, informs us that “This system of siderography continued in use for bank-note printing in the bank of England till 1855, when electrotype-printing was introduced.” It would seem from the Bureau of Engraving site that in America this “iron writing” still continues. The Grolier Club caption also refers to the process as the transfer process. Siderography was perfected in the early nineteenth century by Yankee inventor Jacob Perkins who developed a special soft steel that could be hardened.
The process works thus: the vignette (as the artwork for bank-notes, stock certificates etc. was called) would be engraved in reverse into a flat piece of soft steel called a die. The finished die is then hardened and put onto a transfer press. A wheel-like disk of soft steel is rolled slowly over it at high pressure so that the soft steel is forced into the recessed image on the die. This results in a raised positive impression around the edge of the wheel. This wheel is then hardened at high temperature, and is then put into the transfer press where it presses the image and text into the soft metal of the printing plate. That appears to be what’s going on in the picture at the US Bureau of Engraving site. This plate, having the image recessed into its surface must thus be being used for an intalio form of printing. The Bureau refers to it as offset printing (click on the tab “How Money Is Made” at the left of the page) so I assume we are in the realm of offset gravure.
(For gravure, see A la poupée printing.)
* They also give us the word siderographist, but not -pher, and tell us that a siderograph is “A steel engraving, esp. one produced by siderography”. Incidentally — and one wonders how one has got this far through life without having to call on this impressive word — siderodromophobia is an irrational fear of railways or railway travel. I guess if you have it you’d know it. There are several other sidero- words of course. And just to keep us hopping we are also told that the prefix sidero- also covers a small number of words meaning star shaped.