His lush and naturalistic style earned him many admirers and the opportunity for several exhibitions, including one at the renowned Hunt Botanical Library in Pittsburgh in 1972. Even today, he remains a model and inspiration for many artists of naturalistic painting. His drawings for the nature guide of the KOSMOS publishing house and the BLV book publishing house made him world famous.
Caspari produced several hundred dry watercolours, including mushrooms, minerals, fish, insects, orchids and other plant portraits for Kronen-Verlag Erich Cramer. It was agreed, as with all artists of the Kronen-Verlag, that the acceptance of the drawings would be subject to approval by appropriate scientists from universities and research institutions. Discussions between artists, scientists and the publisher focused on the correct reproduction of the smallest details, be it the fin position of a fish, the bend of a petal or the colour shading of a mushroom. The artists found sketchable models in universities, private collections, or during laborious excursions in Europe and around the world.
Erich Cramer worked closely with universities and private collectors to find or acquire material suitable for drawing and to make it available to the artists. The borrowed minerals, some of them very rare, had to be insured against loss by the publisher, otherwise they were not allowed to leave the storage rooms.
One challenge was that unsatisfactory execution, requests for changes or even rejection of a drawing by the scientists involved or Erich Cramer, often led to a considerable delay in completion.
Nevertheless, Erich Cramer wrote to Caspari during the preparation of the mineral work: "[...] We cannot live on concessions with the mineral work, but with the finished work we must present something that the world has not yet seen in this perfection. [...] I have realised that any forcing will not lead us to the desired goal, and that is why I have already told you that I will take any time pressure off you, so that you have time and leisure to choose the most appropriate and effective lighting for each representation and to apply the greatest possible care in the execution of the drawings. [...] it would be much worse if you and I and the others involved in this work felt afterwards that the result was not up to our abilities and skills, just because we thought we had not had time."
"Claus Caspari is one of the few artists today who still cultivate naturalistic painting and know how to combine artistic expression with scientific exactitude. His admirable art places him worthily alongside a Sybilla Merian, a Moritz Michael Daffinger and Pierre-Joseph Redouté."
Quote Erich Cramer, 1964