Description of the Most Principal Mineral Ores

Assaying furnace simply made of bricks

can be had / and yet in a hurry an assay / or several should be tested / so one can put together an assaying furnace from bricks / in a square / leaving air holes on the sides / and a mouth hole in front / also put a muffle made of half a pot inside / for in such a furnace the test assays can also be correctly performed.

Assaying furnace in which the fire is best to be regulated.

The assaying furnaces / however / in which it is most convenient to assay / and the fire is to be correctly and well regulated / and which do not easily clog with ash / and in which all assays ( that are to be tested in an assaying furnace ) can be made / are to be made as follows / Take the above-mentioned length / according to which the first assaying furnace is divided / and measure into the square eleven units in width / and sixteen units in height / which is the total height of the assaying furnace / And when you have measured eight units in height / so taper the assaying furnace a little / thus / that it remains seven units wide at the top in the square / and its thickness should be one and a half units / and the bottom also made of clay / three-quarters of a unit thick / Then begin above the bottom and measure three units in height / and four and a half units in width / which will give the lower mouth hole / then measure further above the lower mouth hole two units in height / which will give the space between the lower and upper mouth hole / likewise measure three and a half units in height / and four units in width / on this measurement make the upper mouth hole / measure one more width above the upper mouth hole / at which place you should make a hole / a small finger wide / so that from the center of the same hole to the top of the assaying furnace / there remains seven and a half units / Then measure on both sides of the mouth hole / toward the taper three-quarters of a unit / under which measurement / make two holes a reasonable finger wide / which go straight through the furnace / and out the back / When this has been done / you must also on both sides / and at the back of the furnace make a ledge / on which the ash can fall down / which from the bottom should be two and a half units / and also two and a half units from the side / standing inside the furnace / and the ledge should taper inward / from the bottom seven and a half units / Only keep this in mind / when you want to make something of clay / so you must add as much as the clay is accustomed to shrinking / for one shrinks more than the other / but generally the clay shrinks by the tenth part. When this assaying furnace is now made ready / and is still soft / so folds must be cut into it / therein