Does this reflect a collapse in the practica market, or simply less interest in cataloging sixteenth-century books compared to incunabula? Without a baseline comparison, I couldn't definitively answer the question.
One way to find an answer would be to compare the number of editions listed in VD16 for the early sixteenth century with the number of editions from German printers listed in ISTC (not including broadsides, which aren't recorded in VD16; also, there might be several VD16 numbers associated with a single edition if it contains multiple works, so we need to count editions rather than VD16 numbers). If there's a notable decline from the fifteenth to sixteenth centuries, then the apparent decline in practica editions is more likely to reflect different bibliographic standards. A further complication is that a lot of undated editions get dated to round years like 1495 or 1500, so we're particularly interested in dated editions.
Here's what we find:
The dated ISTC and VD16 records of German printing between 1500 and 1501 are almost perfectly continuous. If we ignore the spikes in undated additions ascribed to 1490, 1495, 1500, and 1505, the overall record also has very little gap.
So the decline in practica printing looks more and more like a market collapse rather than a bibliographic artifact. Something very similar happens between the 1590s and the first decade of the seventeenth century. In other words, the market for predictions looks much stronger just before the turn of a century than just after, just like the market for Y2K hysteria was much better in 1998 than in 2002.
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I have a few pressing events coming up. Posts may be infrequent for the next several weeks.