Cornelis was a very successful entrepreneur and that allowed him to become one of the most important art collectors from the Netherlands. Besides collecting drawings he was also interested in the scientific aspect of print making and experimented constantly in seeking new techniques to make etchings in a drawing alike style that reproduced his important collection. He experimented with printing in colour from several plates. He can be called a real inventor and brought the technique of printing to a new level. His working methods have been revealed by Th. Laurentius and catalogued in the work 'Cornelis Ploos van Amstel' Kunstverzamelaar en prentuitgever, published in 1980.

He worked with other printmakers who were obliged to keep the newly invented techniques secret. Some of his assistants include Bernard Schreuder, Elisabeth van Woensel, Cornelis Buys, Cornelis Brouwer. His printer was Johannes Körlein. Son-in-law of Cornelis Troost

Bernard Schreuder (active for Ploos between 1767-1778 - died 1780)

Ploos van Amstel hired schroeder in 1767 with the terms that he had to be discrete about the secret techniques he learned from the master. After several rejected proof states, only in 1774 after 7 years, the first etching entirely done by Schreuder was published. In 1778 the cooperation between Van Amstel and Schreuder ended as Schreuder became more and more arrogant and difficult. Schreuder then continued to make print drawings and published his own collection of print drawings to set foot against his former employer.

The prints by Schreuder below do not belong to the 'Ectypa' by Ploos
 it are works made besides this or after his cooperation with Ploos ended.