Second, in slags, glazes and ,glasses addition of Al208 above a given amount increases their viscosity, but no limiting points have, as yet, been determined except in the case of slags. Since slags are comparatively simple in composition and usually relatively high in lime, we can learn very little by reviewing in detail the researches that have been made on the vicosity.
Third—Increase of Al208 in small amount in glasses increases their toughness. So far as data have been obtained increase of Al203 in por celain bodies does not increase their toughness.
From these conclusions a query is at once presented concerning the relation between fusibility, viscosity and toughness. At present any dis cussion of this query would be based wholly on assumption, for there are no experimental data bearing on the point.
Effect of Silica in Ceramic Mixtures—Anhydrous silica is practically inert at ordinary temperatures, but at the temperature usually attained in brick kilns it becomes very active, forming compounds having very varied oxygen ratios, i. e., amount of oxygen in the basic to the oxygen in the acid oxides.
On heating, silica expands considerably, indicating peculiar molecu lar changes. LeChatelierz has shown that at 500°C. this molecular change takes place to a very pronounced degree in all forms of silica, the least in amorphous and the most in highly calcined flint. Per manent expansion in highly silicious bricks and the "punkness" of bricks made from a mixture of clay and sand are evidence of the effect of this peculiar property of silica.
No matter how fine the free silica is, it does not seem to be as active in forming new silicate compounds under the influence of heat as is the silica that is previously combined, as for illustration, in clay or feldspar. In other words, silicate combination with free basic elements is affected more readily when the silica is added to the mixture as a constitutent of a pre-existing silicate. This was shown very prettily in an experi ment reported by Bleininger.' He prepared a mixture of 20 per cent finely ground flint and 80 per cent precipitated calcium carbonate and two other mixtures each containing respectively 20 per cent 'finely ground feldspar and 20 per cent of kaolin with 80 per cent calcium car bonate. These mixtures were maintained at a temperature of 1100 C.
for 75 minutes. At this temperature calcium silicate compounds are formed which are soluble in hot hydrochloric acid and sodium carbonate solutions. The residue left after this acid and alkali treatment is the material which is unattacked or unlocked by the fluxing action of the' lime. In the following table are Bleininger's results.
Bleininger's results strongly support the doctrine that has, for the sake of emphasis, been repeatedly stated in this report, to-wit : That very little can he told concerning the fusing behavior of silicate mix tures from an ultimate analysis, for if this were not the • case, feldspar should have reacted far more vigorously with calcium carbonate than did clay. Since cement investigators have found that the hydrous am orphous silica reacts with lime in a manner similar to finely pulverized crystalline quartz, it can be readily seen that misleading data would he obtained even in the rational analysis, in which the hydrous amorphous silica is taken into solution by the sulphuric acid and thus considered as a part of the clay substance.
Addition of silica to pure clays like shales increases their ness and, (reasoning from data on slags) possibly, their viscosity. There is no evidence showing that the addition of flint to a clay increases its toughness, but quite the contrary, empirical experiments by several prac tical brick manufacturers have proved that the additon of ordinary bank sand makes the bricks less tough or even very "punky." On the other hand an investigation by proved that Bedford shale, which outcrops near Columbus, 0., is materially benefitted by an addition of crushed Berea sandstone from the same locality. Instances are recorded of addition of certain sands in Europe having proved beneficial, but in neither Worcester's experiments nor in the European cases was there reported a determination of the effect of sand on the toughness of the burned mixtures.
In the manufacture of floor tile the writer found that a porcelain body consisting of 40 per cent clay, 45 per cent feldspar and 15 per cent flint was much tougher than a body containing 35 per cent clay and 65 per cent feldspar. It is impossible to say why the body containing flint should be tougher but certainly some credit must be given to the influ ence of the flint.