Description of the Most Principal Mineral Ores

high / and on the other three sides / at the bottom of each there was a wind hole / four units long / and one and a half units high / also the iron plate had holes punched throughout / so that it was rough and sharp / so that the clay could adhere when lining the furnace on the inside. For the lining / however / of such an assaying furnace / a particularly good clay is made / which holds up well in the fire / as follows / Take good well-tempered clay / mix into it shear-wool or horse manure / ox blood / iron hammer-scale / and common salt / and with this line the furnace / two and a half units thick / let it dry / then take finely ground Venetian glass / bone ash / and a little thin clay / mixed together / smear the lined furnace with this on the inside / and let it dry well / make a steady fire inside / so that it glows. When one now wishes to use this furnace / one makes a level hearth / and on the hearth a small thin hearth-plate / of cupel ash / one cross-finger thick / on top of this one sets the furnace / and inside the furnace on the small hearth-plate a muffle / which is shaped as the following figure will show / thus the assaying furnace is ready. When testing is now done in the assaying furnace / it glazes itself on the inside / from the final coating / with Venetian glass and ash / and lasts all the longer / also when the furnace burns away completely in length / one can knock out the old clay / and line it with new / just like at the first / In such a furnace / whoever understands the regulation of the fire well / or is a practiced assayer / can make all necessary assays therein / only the wind holes clog themselves much sooner with the ash / than the furnace with the two mouth holes / about which a report will follow later.

Nuremberg assaying furnaces / of potter’s clay.

Some also use for their assays / furnaces that are made of potter’s clay / and shaped like the furnace just described / they are bound with wire / and stand on a foot / which is wide and hollow / and has round wind holes on all four sides / and inside above in the furnace also wind holes / as the following figure will also denote this furnace / In this furnace the fire is less to be regulated / than in the above-mentioned assaying furnace / for the wind holes in that same furnace / which are cut into the foot / clog very easily / and such assaying furnaces are called Nuremberg assaying furnaces / and muffles belong inside them / the form of which can also be seen in the figure.

So if one is in a place / where one has no assaying furnace