For every new work in our collection, the question arises concerning the biography of the artist as well as the history of the work. Particularly with works of high value, the second question must be clearly answerable. Buying such works without solid provenance constitutes a significant risk. Should it turn out that a work changed hands from its owner during an expropriation action in the Nazi era, or if the owner was forced to sell under duress, it is considered looted art and subsequently unsalable.

While that would not have any immediate significance for us, as we no longer sell our works, such a case would nonetheless create a considerable moral obligation for us to find a fair settlement with the heirs.

The Washington Conference Principles on Nazi-Confiscated Art from December 3, 1998, provide guidelines for this. While these are not legally binding, they are morally and politically binding, which means the states that joined this agreement have obligated their state museums to research the provenance of works that came into their possession during or after the Nazi era.

In a few cases involving works we very much wanted to purchase, we managed to clarify a missing provenance at least to the point that the probability of it being looted art was minimal. This was not entirely straightforward, however, as you are primarily dependent on an auction house asking the work's consignor if they agree to an outing to a potential buyer. Because this step occurs before the auction, auction houses react very differently to such inquiries. If you cannot achieve this first step, you should not buy such a work.

During our research, we found that almost all works came up for auction every 20 years—namely, after the previous owner's death. The heirs often cannot do anything with them and consign the work to auction. And it is actually statistically proven that most works remain in a collector's possession for around 20 years.

Often, stickers or written notes on the back of the frame also provide valuable clues. That is why for us, looking at the reverse side is just as interesting as the front. If there are, for example, stickers from previous auctions or galleries where the work was sold, you have good starting points.