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524
BOOK XI
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and forth. This bar is round, eight feet long and two digits in diameter ;
on the right side it has a haft made of iron, which is about a foot from the right end ; the aperture in this haft is a palm wide, two digits high, and a digit thick. The bar holds the exhausted liquation cakes opposite, lest they should fall down. When the operation of " drying " is completed, a workman draws out this bar with a crook which he inserts into the haft, as I will explain hereafter. In order that one should understand those things of which I have spoken,
and concerning which I am about to speak, it is necessary for me to give some information beforehand about the furnace and how it is to be made. It stands nine feet from the fourth long wall, and as far from the wall which is between the second and fourth transverse walls. It consists of walls, an arch, a chimney, an interior wall, and a hearth ; the two walls are at the sides ; and they are eleven feet three palms and two digits long, and where they support the chimney they are eight feet and a palm high. At the front of the arch they are only seven feet high; they are two feet three palms and two digits thick, and are made either of rock or of bricks ; the distance between them is eight feet, a palm and two digits. There are two of the arches, for the space at the rear between the walls is also arched from the ground, in order that it may be able to support the chimney; the foundations of these arches are the walls of the furnace; the span of the arch has the same length as the space between the walls ; the top of the arch is five feet, a palm and two digits high. In the rear arch there is a wall made of bricks joined with lime ; this wall at a height of a foot and three palms from the ground has five vent-holes, which are two palms and a digit high, a palm and a digit wide, of which the first is near the right interior wall, and the last near the left interior wall, the remaining three in the intervening space ; these ventholes penetrate through the interior of the wall which is in the arch. Half-bricks can be placed over the vent-holes, lest too much air should be drawn into the furnace, and they can be taken out at times, in order that he who is " drying " the exhausted liquation cakes may inspect the passages, as they are called, to see whether the cakes are being properly " dried." The front arch is three feet two palms distant from the rear one ; this arch is the same thickness as that of the rear arch, but the span is six feet wide ; impurities ; the " ash-coloured copper " to contain about 60% cuprous oxide and 30% lead
oxide, with some metallic copper and minor impurities. An average of proportions given by various authors shows, roughly, that out of 100 centners of " exhausted " liquation cakes, containing about 70% copper and 30% lead, there were about 63 centners of " dried " copper, 38 centners of " slag," and 6£ centners of " ash-coloured copper." According to Karsten, the process fell into stages ; first, at low temperature some metallic lead appeared; second, during an increasing temperature for over 14 to 15 hours the slags ran out ; third, there was a period of four hours of lower temperature to allow time for the lead to diffuse from the interior of the cakes ; and fourth, during a period of eight hours the temperature was again increased. In fact, the latter portion of the process ended with the economic limit between leaving some lead in the copper and driving too much copper into the " slags." Agricola gives the silver contents of the " dried " copper as 3 drachmae to 1 centumpondium, or equal to about 9 ozs. per ton ; and assuming that the copper finally recovered from the bye-products ran no higher, then the first four charges (see note on p. 506) would show a reduction in the silver values of from 95 to 97% ; the 7th and 8th charges (note on p. 512) of about 90%. |
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