The Archives of the Kolff Family are included in the Stadsarchief Rotterdam (name changed from Gemeentearchief Rotterdam).
You can check the inventory at the website of the Stadsarchief Rotterdam. The introduction pages are in English and Dutch, deeper inside the website information is available in Dutch languege only.
Do this as follows (follow the steps as indicated below):
– open collections
– open archives
– from here you will have to follow the Dutch language pages,
– so first now choose for ‘Dutch language’ in Search Archives, and then follow below steps
– choose within that page for ‘Systematisch Archievenoverzicht’ (middle of that page)
– open list ’18 – Huizen, Families, Personen’
– then choose ‘2 – Families’
– in the overvies you will find ‘1310 – Archieven van de Familie Kolff’
(scroll through the list, numeric order)
– clicking on that link opens the page of this archive.