REFRACTORIES FOR GASWORKS PURPOSES 207 completely covered, the pump sucking away the air from the interstices of the brick, paraffin taking its place. By levelling and reading the burette, the volume of liquid displaced by the solid portion of the test piece will be given. The volume of the soaked brick is then foimd by displacement in a similar manner, thus giving the total volume of the test piece, and the percentage porosity calculated as above.
AFTER CONTRACTION AND EXPANSION
When fireclay articles are burned in the kilns a certain amount of contraction occurs ; but usually such contraction is not wholly completed during this operation, with the result that further contraction takes place when tlie articles are in use. The secondary change in volume is generally known. as after-contractioii or “ after-expansion,” and it will be appreciated that, if the stability of a construction such, as a retort setting is to be ensured, effects of the kinds must be eliminated as far as possible. Mellor 1 states that when firebricks are kilned the fluxes melt and dissolve the less fusible materials, binding the whole more or less together.
It is well known that ferrous iron has a far greater fltixing action than ferric iron, so that the contraction of a brick fired under reducing conditions will probably be greater than when fired under oxidizing conditions, providing no bloating occurs. Nearly all firebricks are burned under oxidizing conditions. It therefore appeared of interest to find the difference in the results obtained in the specification contrac-tion test when conducted under oxidizing and under reducing conditions. Some measurements are given in the following table 2 :—
Percentage after Expansion or Contraction. (Means of Duplicates).
Type of Brick. Oxidizing Atmosphere. Reducing Atmosphere.
Silica brick, unusually fine grained .... 0-44 C 0-77 C
Silica brick, high in “iron” 0-58 E 0-48 E
Mrebrick 0-21 E Nil —
Firebrick Nil — 0-33 c
Firebrick 1-12 c 1-27 c
Silica brick 0-20 E 0-13 E
Silica brick 0-13 E 0-12 E
C signifies contraction ; E, expansion.
These results show a general tendency for silica bricks to give a less expansion in a reducing atmosphere than in an oxidizing atmosphere ; and for aluminous firebricks to give a greater contraction in. a reducing than in an oxidizing atmosphere.
1 Proc. Faraday Society, November, 1916.
2 Inst. Gas Eng. Refractory Materials Committee, 1917.