..in order to etch, take white turpentine oil, and add half the turpentine to it; pour the mixture into a small glass bottle and let it boil in pure [?] water for half an hour.

It is evident that Rembrandt refers (alas fragmentarily) to a so-called 'stopping-out varnish', used to terminate the bite of acid in select areas of a plate that had already been exposed to the etching agent. Thus other portions will remain exposed to the acid to deepen the bite. Also Samuel van Hoogstraten, the first student of Rembrandt in Amsterdam, mentions the use of such a varnish in his 'Inleyding tot de Hooge Schoolde der Schilderkunst', Middelburg 1671 Rotterdam 1678