This revival appears to have paralleled in general the political and cultural revival, but, as in any mining region, the exhaustion of easily workable surface deposits marked a critical point, when the necessity of deeper mining led to the construction of supported tunnels and the introduction of machinery for removing ores and water from deep mines. On the basis of revisions of capital structure and mining law which he regards as inspired by the financial necessities of deep mining, Bechtel dates this development from the mid-14th century.[4] The mid-14th century situation is confused by the occurrence of the Black Death, which reduced mining activity drastically, and the events of which Bechtel speaks have been put as much as a century later.[5] In any case, the development of deep-mining methods had clearly made considerable progress in nonferrous mines when the _De re metallica_ was written, in 1556. [Illustration: Figure 1.--BRUNSWICK SILVER 3-1/2 TALER, Johann Friedrich, 1677. (_U. S. National Museum, Paul A. Straub coll.; Smithsonian photo 43334-C._)] MINE-PUMPING MACHINERY ILLUSTRATED BY BRUNSWICK MULTIPLE TALERS These large silver coins weighing up to 15 ounces were first issued in 1574 in Brunswick by Duke Julius (1568-1589) of the Wolfenbuttel line. Their historical background is rather unusual and interesting. In 1570 the Duke decided to increase the output of his silver mines in the Harz and arranged for the opening of three new mines. In order to insure the retention of a portion of this increased silver output under his control, the Duke decided to issue an entirely new kind of silver coin which he called "Loeser," meaning redeemer. These were larger than taler-size pieces, and were struck in denominations from 1-1/4 to 16 talers. The Duke ordered that each of his subjects was to purchase one of these large coins, the size of the coin to be acquired depending on the individual's wealth. The owners were not allowed to use these pieces in everyday trade, but could pawn them in case of dire need. They were expected to produce them at any time upon demand. Thus a means of hoarding, a "treasure piece," was created, and the risk of draining the country's wealth through replacement of good, full-weight silver coins with imported base currency was to some extent limited. At the same time, the Duke had a considerable sum of money at his disposal in case of emergency.