For the northern districts at least Herr von Czihak has brought forward much evidence to show that the artists in the local towns carried back to the mountain furnaces, to be there fired, the glass that they had painted with enamel colours, and that even the finer kinds of engraving were done in the upland villages where water-power was abundant. This was certainly the case in later days in the famous centre of glass-engraving that grew up at Warmbrunn, in the Hirschberg district of Silesia. On the other hand much glass was, it would seem, enamelled in Dresden, and in the south the finer work both of the enameller and the glass-engraver was probably executed in the studio of the artist—at Nuremberg, for instance, or in other Franconian or Swabian towns. For the German glass of the sixteenth century we have fortunately the two already mentioned contemporary writers, both of them Saxons by birth—Georg Agricola and Johann Mathesius. Agricola, it is true, ‘the founder of the sciences of mineralogy and metallurgy,’ in his famous work _De Re Metallica_,[192] devotes only a few pages at the end of his last chapter to the subject of glass; but here may be found the first accurate drawing of a glass furnace that has come down to us. Agricola mentions that he had passed two years at Venice, and had seen much of the glass-working when there.[193] Indeed, what he says of the materials, of the source of the alkali above all, seems to have relation to the Italian rather than to the German glass. [Illustration: _PLATE XXXVIII_ GERMAN GLASS FURNACE SIXTEENTH CENTURY. FROM AGRICOLA] But this is not the case with the furnaces, which he describes and illustrates. Agricola distinguishes three separate ovens: the fritting oven; the main oven, where the glass is melted in pots; and an annealing oven for slowly cooling the glass. These ovens, however, may be combined in various ways in smaller works, reducing the number to two or even to one. The fritting oven is a detached building of beehive shape, which is also used for annealing the pots. The main oven, eight feet in height and ten feet in diameter, is of a similar outline. The wood is burned on the floor of a lower chamber, without any grating of firebars; the flame passes through into an upper chamber, around which are arranged eight pots, each two feet in height, with a working-hole in front of each pot. From the back of this chamber a passage opens which conveys the heated gases to the quadrangular annealing oven.