Sir William Congreve was in his element as a member of the Royal Commission. He was a prolific inventor, having obtained patents ranging from a military rocket system (for which he is perhaps best known), through to gun carriages and even the construction of locks and sluices for waterways. Congreve has been described as ‘self-important and an avid publicist for himself…’  and ‘a military martinet of the old school…’ . These character traits come out in his work on compound-plate printing. Congreve’s ambiguous position as both Royal Commissioner and inventor gave him a commanding position in his field. He had an overview of all the 108 submissions for reducing forgery received by the Commission, while developing his own scheme, which he patented in 1820 .
Congreve’s scheme was outlined in an 1820 pamphlet . It comprised two main processes. First, the banknote printing plate would consist of two parts: the upper one would have shaped holes in it, into which identical protuberances in the lower one fitted. Once interlocked together, the two parts would be engraved simultaneously with complex geometric patterns that overlapped across both of them. Secondly, when in use on a specially devised printing press, each plate would be inked in a different colour before being brought together to produce a two-colour image from a single impression. So, the process was ‘compound’ in two senses: it used pairs of plates to make two-colour images but, most importantly, its strength came from employing both geometric patterns and multiple colours in such a way that the two elements could not be used separately to achieve the same result. In Congreve’s words, the process was ‘indivisible’ .
Compound-plate printing posed a formidable technical challenge not only to would-be counterfeiters but also to Congreve himself. To implement the scheme, he engaged Bryan Donkin, a major figure in London’s booming engineering industry at the time. Donkin (1768–1855) was prolific, working in fields as diverse as food canning and paper-making. He had developed the first continuous paper-making machine for the Fourdrinier Brothers at Bermondsey and, when they went bankrupt in 1810, he took their works over. In 1818 Donkin had also patented a flat-bed printing machine capable of dual-colour reproduction . He was the ideal candidate to make Congreve’s compound-plate printing a practical reality.
If Donkin was ideally suited to assist Congreve, two others were well-placed to compete with him: Jacob Perkins and Asa Spencer. Perkins had arrived in England from America in 1818. His intention was to persuade the Bank of England to adopt his banknote production process, which he called ‘siderography’ . This process entailed the engraving and case-hardening of a steel plate, which could act as the master die for duplicate printing plates manufactured from the master using a plate transfer press . To this end, Spencer bought with him to the UK a complete set of machinery and a number of machinists and engravers, including Spencer. In 1815 Spencer, a watchmaker from Connecticut, had transferred the rights to a geometric lathe he had invented to Perkins. When Perkins patented his process in England in October 1819, it included details of Spencer’s lathe.