Thomas cottrell, described by Mores as à primo proximus of modern letter-founders, served his apprenticeship in the foundry of the first Caslon. He was employed there as a dresser, and the portrait of him which is to be seen in the Universal Magazine of 1750, among a group of Caslon's workmen, represents him as engaged in that branch of the business.

It is not improbable that he joined with his friend and fellow apprentice, Joseph Jackson, in clandestinely observing the operation of punch-cutting, secretly practised by his master and his master's son at Chiswell Street; and being assisted by natural ability, and what Moxon terms a “genuine inclination,” he contrived during his apprenticeship to qualify himself not only in this, but in all the departments of the art.

In 1757 a question as to the price of work having arisen among Mr. Caslon's workmen, Cottrell and Jackson headed a deputation on the subject to their employer, then a Commissioner of the Peace, residing at Bethnal Green. The worthy justice taking this action in dudgeon, the two ringleaders were dismissed from Chiswell Street, and thus thrown unexpectedly on their own resources.

Cottrell, in partnership for a short time with Jackson, and (according to Rowe Mores), assisted also by a Dutchman, one Baltus de Graff, a former apprentice of Voskens of Amsterdam, established his foundry in Nevil's Court, Fetter Lane.