In 1657 Ahasuerus's son John Fromanteel began studying pendulum clocks, invented by Christiaan Huygens. Before the invention of the pendulum clock, timepieces were accurate to only within ten to fifteen minutesa day. The use of the pendulum made for near frictionless time keeping, ensuring that the mechanism lost measurement of only a few seconds a day: a sixty-fold improvement. It was termed a "horological breakthrough". This revolution in time keeping could be said to have caused industrial espionage on a grand scale. Although claimed by Samuel L. Macey to have caused "industrial espionage on a grand scale" it has been argued by Theodore M. Porter, that Macey fails to conform with the normal writings associated with history. That Macey can be accused of rambling and quite often losing the point of an argument he is trying to make. Therefore there is not much academic scholarship to be associated with Macey's argument that Fromanteel was guilty of any form of espionage. Following a trip to the Netherlands John and Ahasuerus implemented the new, more accurate pendulum technology. Fromanteel became the first maker of pendulum clocks in England, although this distinction has also been claimed by horologists Richard Harris and Robert Hooke. Precise measurement of time was critical for disciplines including maritime navigation and astronomy. Fromanteel used the design for regulating steeple and domestic clocks and sold them from the family house in Southwark and at the a shop in Lothbury, which had been a famous retailer of clocks for more than a hundred years. The Fromanteel family sold also weight-driven clocks, watches that needed only a single annual winding and a variety of domestic and industrial engines, selling to the city of Norwich one of his patent fire engines. In the Commonwealth Mercury of Thursday 25 November 1658 Fromanteel advertised :
clocks that go exact and keep equaller time than any now made without this regulator and are not subject to alter by change of weather, as others are, and may be made to go a week, a month, or a year with once winding up, as well as those that are wound up every day, and keep time as well, and is very excellent for all House Clocks that go either with springs or weights; and also Steeple Clocks that are most subject to differ by change of weather. Made by Ahasuerus Fromanteel, who made the first that were in England. You may have them at his house on the Bankside, in Mosses Alley, Southwark and at the sign of the Mermaid, in Lothbury, near Bartholomew Lane end, London.
He had gained the notice and patronage of "The Lord Protector", Oliver Cromwell, Britain's interregnum head of state. A popular legend developed that Cromwell himself owned such a clock, however there is little evidence to support this theory. As Mark Denny states in Five Machines That Changed the World, Fromanteel's patent "ushered in the age of English longcase clocks, which dominated horology for a century".
Later years
Fromanteel relocated back in the Netherlands in 1667-76. His son John was left in London to look after the business with Thomas Loomes, a former apprentice, married to Fromanteel's daughter Mary. Following the death of his wife Maria, Ahasuerus Fromanteel wed the widow Sarah Winnock. They lived in Whitechapel, where other of the Fromanteel family were established as silk weavers. Fromanteel died in 1693, and was interred at St Mary Matfelon, Whitechapel, on 31 January 1693. This church was destroyed in the Blitz, and Fromanteel's grave has been lost. A Fromanteel brand-name of clocks and watches exists today, based in the Netherlands.
