He was a peintre-graveur who preferred the spontaneity of the needle over the brush. Zilcken often worked directly on the copper plate while outdoors to capture the immediate atmosphere of a scene. This approach is evident in his portraits and animal studies, such as Leeuw or Hoofd van een oude vrouw. He used strong, confident lines to define form without relying on heavy shading. His work prioritised the emotional presence of a subject rather than a literal photographic representation.

Modern collectors value Zilcken for his ability to combine traditional Dutch realism with a global perspective. His etchings offer a quiet, contemplative quality that suits contemporary interior spaces. The monochromatic nature of his work provides a sophisticated look that complements various decor styles. His depictions of daily life, from a Boer in a field to rural views of mills, remain relatable because of their focus on character and simplicity.