Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/250

 240 F I B F I B converting it into anhydrous sesquioxide, to which the yellow colour of the burnt brick is due ; if the burning is cirried to a high state of vitrification the ysllow tint is replaced by a dull grey, due to the partial reduction of the sesquioxide of iron and its conversion into silicate of protoxide or minutely disseminated particles of metallic iron; any alkalies present also form vitreous combinations with the silica during the latter stages of the burning. The paleness of colour of a fire-brick is not always a safe indica tion of the absence of iron, as the presence of a large proportion of carbonaceous matter in the clay tends to bleaching by the reduction of the colouring sesquioxide to a lower oxide preserved as a silicate in a comparatively colourless condition. Again, the presence of lime and the other alkaline earths, which are disadvantageous fluxing elements, will cloak the colouring power of a large per centage of oxide of iron by the formation of a pale double silicate of lime and iron. This is taken advantage of in the manufacture of buff building bricks by mixing ground chalk with ferruginous clays which would otherwise burn dark red. A properly burnt brick, uniform in colour throughout its mass, can only be obtained by slow progressive firing ; a broken brick that has been too quickly burnt, though pale on the surface, presents a darker central patch and con centric rings of various shades of colour, due mainly to the different states of oxidation of the iron, and partly to the presence of unconsumed carbonaceous matter ; but the chemistry of this colour-variegation is not clearly under stood. Durability of Fire-Bricks. The destruction and wearing out of a fire-brick in its ordinary uses takes place in different ways. First, it may waste by crumbling and shattering; this occurs only when the brick is unnecessarily porous in texture, or from the presence of extraneous lumps of foreign materials, such as small pebbles and fragments of lime and iron-stone, which the manufacturer endeavours to get rid of by sifting or crushing before the clay is moulded. Secondly, the gradual vitrification of the brick under the pressure of the superimposed structure distorts its form, and the semiplastic red-hot or white-hot mass is gradually squeezed out of shape, and has to be periodically replaced in the hotter parts of the furnaces and kilns. However completely a brick has been burnt, bringing its dimensions almost to the limit of contractility, constant exposure to long-continued heat still further reduces its bulk, causing the displacement of the mass of which it forms a part, and necessitating replacement and repairs. Thirdly, there is the gradual fretting away of the exposed brick surface by vitrification, however refractory a brick maybe; when it lines flues and furnaces, the fumes and ashes incessantly cirried into contact with it bring foreign accessions, which vitrify the exposed portions and form a coating of viscid slag, which eats into the brick surface, creeping down and clogging the flues and fire-holes with a vitreous mass. In the case of blast-furnaces the fretting away of the surface of the fire-brick linings gradually enlarges their capacity, the surface destruction decreasing from the tweers upwards, the faces opposite the impact of the blast being distinctly excavated beyond the outline of the enlarged circum ference. In one of the furnaces of the Madeley Wood Company, Madeley, Shropshire, &quot; blown in &quot; in 18G7 and 3 feet, had been enlarged to about 7 feet G inches, and at the widest part, a little below the middle, from 12 feet, its original diameter, to 13 feet. In another furnace, after ten years blast, the original diameter of 4 feet 5 inches at the base had been enlarged to 9 feet G inches; at a third of its height from 9 feet 11 inches to 12 feet; and half way up, from 11 feet 9 inches to 12 feet 10 inches, the destruction of brick-surface gradually decreasing toward? the top, where the increase of size was but trifling. This would represent a consumption of brick-surface of from 3 to 4 inches a year at the tweers, and about three-quarters of an inch a year towards the middle of the furnace. It is beyond the scope of this article to enter into the details of fire-brick manufacture, which in its main features resemble the manufacture of building bricks, except that fire-bricks are rarely if ever burnt in clamps. Properly constructed ovens on the &quot; down draft &quot; prin ciple, with the outlet from the bottom into a tall chimney, are now almost universally employed, as they ensure greater regularity in the burning than in the old form of kiln, with a direct escape from the top, as well as economy in fuel. The consumption of coal varies from 9 to 15 cwt. per 1000 bricks, exclusive of the coal used in the drying stoves. Coal-Measure fire-clays are often mined in an almost rocky condition, requiring long exposure to the weather to effect their disintegration. The softer clunches or clays are sometimes prepared for the moulder by &quot; weathering,&quot; but the more common practice is to grind the tire-clay just as it comes from the pit under heavy &quot; runners &quot; or rollers, effecting a granular texture which is a desirable quality in a refractory brick. It is now becoming a com mon practice to grind up with the raw clay from one- fourth to one-third of its weight of broken burnt sherds or fire-bricks. All waste materials are thus utilized, and the excessively contractile character of highly plastic clays such as those of Dorsetshire is counteracted. Silicious sand is also sometimes mixed with the more plastic clays to reduce their contractility. The ground clay is either brought into a plastic state with water in a pug-mill and moulded by hand, or by brick- moulding machinery generally connected with the pug-mill outlet ; or the partially moist ground-clay dust is com pressed into bricks in iron moulds by steam power, a modification of Prosser s well-known process. More shapely bricks are thus produced than by plastic moulding, and their perfectly true flat sides enable a minimum of joint ing materials to be employed a circumstance of import ance in the stability of fire-brick masonry, as thick fire clay jointing contracts in the firing, tending to shatter the structure. In addition to the use of fire-clay for the bedding of all fire-brick structures, it forms the materials of gas retorts, crucibles, and every kind of potter s kiln furniture, such as saggars, cranks, slip kiln bottoms, enamel kiln linings, &c. These have been up to within the last year or two moulded out of plastic clay, but by processes and machinery recently patented by Mr A. Maw, saggars, cranks, and every kind of kiln furniture can be moulded out of nearly dry pulverized clay, at a great reduction in cost below that of the plastic process, and the exact regularity of form attained effects a large saving of space in the ovens and kilns. Moreover, the even bearing on each other of the regularly- shaped saggars when piled up in the oven, reduces the &quot; wear and tear &quot; and breakage to a minimum. Fire-bricks vary much in price in the different producing districts, about 40s. per 1000 may now be looked upon as a minimum, ranging up to 70s. and 80s., further augmented by the cost of carriage to other districts. A thousand fire-bricks weigh from 3 to 4 tons. (G. M.) FIRENZUOLA, AGNOLO (1493-c. 1545), Italian poet and litterateur, was born at Florence, September 28, 1 493. The family name was taken from the town of Fireuzuola, situated at the foot of the Apennines, its original home. The grandfather of Agnolo had obtained the citizenship of Florence and transmitted it to his family. Agnolo was destined for the profession of the law, and pursued his studies first at Siena and afterwards at Perugia. There he
 * blown out&quot; in 1874, the diameter at the base, originally