280 
Steel. 
The straw colour denotes a proper temper, either for 
a razor, graver, or pen-knife. Spring knives require a 
dark brown ; scissars a light brown or straw colour ; 
forks or table-knives a blue. The blue colour marks the 
proper temper for swords, watch-springs, or any thing 
requiring elasticity. The springs for pen-knives are 
covered over with oil before they are exposed to the fire 
to temper. 
Explanation of the Plate . Fig. 1. is a plan of the 
furnace, and fig. 2. is a section of it taken at the line AB, 
The plan is taken at the line CD. The same parts of the 
furnace are marked with the same letters in the plan and 
in the section. EE are the pots or troughs into which 
the bars of iron are laid to be converted. F is the fire- 
place ; P the fire-bars ; and R the ash-pit. GG, &c. are 
the flues. HH is an arch, the inside of the bottom of 
which corresponds with the line IIII, fig. 1. and the top 
of it is made in the form of a dome, having a hole in the 
centre at K, fig. 2. LL, 8tc. are six chimneys. MM. is i 
a dome similar to that of a glass-house, covering the whole. 
At N there is an arched opening, at which the materials 
are taken in and out of the furnace, and which is closely 
built up when the furnace is charged. At OO there are 
holes in each pot, through which the ends of three or four 
of the bars are made to project quite out of the furnace. 
These are for the purpose of being drawn out occasional- 
ly to see if the iron be sufficiently converted. 
The pots are made of fire-tiles or fire-stone. The bot- 
toms of them are made of two courses, each course being 
about the thickness of the single course which forms the 
outsides of the pots. Th^ insides of the pots are of one 
course, about double the thickness of the outside. The 
partitions of the flues are made of fire-brick, which are of 
different thicknesses, as represented in the plan, and by 
dotted lines in the bottom of the pots. These are for sup- 
