The specification of the Institution of Gas Engineers for fireclay bricks includes the following :-

Refractoriness for grade 1 material. Material which shows no sign of fusion when heated to not less than 1670 c. For grade 2 material.-A material which shows no sign of fusion when heated to not less than 1580 C. The test to be carried out at a specified rate of heat increase, and in an oxidising atmosphere.

The specification for Silica bricks deals with material divided into two classes :-

(a) ;That containing 92 per cent, and upwards of silica, called silica material.

(b) ;That containing 80 to 92 per cent, of silica, **nd called siliceous material.

The test pieces are to show no sign of fusion when heated to the following : Silica material to not less than 1690 C. and the Siliceous material to not less than 1650 C. The tests are to be carried out under the heat conditions as mentioned above.

The colour of heated firebrick at different temperatures may be taken as :-

Refractory Cements

Firebrick linings of fireclay bricks are frequently used for many purposes, and the mortar or jointing material between them composed of fireclay. The condition then arises that the firebricks, which have already been burnt before being built into the furnace, are expected to be united in a proper fashion by a mortar of fireclay which has not been burnt. Both are then subjected to the same heat with the result that if the heat does not rise high enough, or until it does rise high enough, the fireclay jointing is not burnt hard, and does not really form one with the firebrick.

The jointing does not bond with the firebrick until it is burnt, the fireclay may therefore dry and powder out, be blown away from between the bricks, leaving the edges unprotected. As the weak spot in all fire brick construction is at the joints of the brick work exposed to the cutting action of the flames and gases, it is evident that the end of the life of the brick lining is in sight when the edges of the bricks are unprotected and begin to disintegrate. A fireclay mortar is merely a space-filling agent, and not a bond, until the temperature is reached at which the fireclay is burnt. It is even yet not fully realised that the life of an expensive firebrick lining depends very much indeed upon the excellence or otherwise of the jointing between the bricks, and to the protection of the brick edges and joints from the flames and gases. There are now excellent refractory cements upon the market, which will set and form a bond with the firebricks at atmospheric temperatures, and which will withstand any temperatures to be met with, at the same time possessing strength and wearing properties, and ability to meet either acid or basic, or neutral conditions. Such cements may be used as mortars or as washes over the brick faces; they should be as nearly similar as possible in properties to the firebricks, and not appreciably less fusible; they should shrink as little as possible.

The cements may include such materials as fireclay, raw or burnt, asbestos, silica rock, lime, portland cement, sodium silicate, china clay, borax, chrome iron ore, quartz, carborundum, magnesia, furnace slag, salt, etc.

The bricks for a furnace lining should have the joints between the bricks as thin as possible, the instructions given with the particular cement should be carefully followed as regards the mixing or use of the same, the bricks when the faces concerned have been thinly coated should be hammered well into position, and the joint edges receive particular attention. These refractory cements are put up either in plastic form ready for use, or in powder form to have water added on site, some of the cements set quite quickly, so that too much should not be exposed at one time before being required. With most of these cements a bonding material is required and this is generally either a small proportion of sodium silicate.

Sodium silicate is commonly known as water glass. Unless a bonding material as mentioned above is used, a cement is generally not strong enough until fusion has occurred or partly so, hence the addition. The manufacturers of refractory cements naturally do not make the composition or method of making known, but the following will demonstrate that the composition does vary widely in known cases, such compositions include :-

(a) ;Magnesia 4 parts, iron turnings 1 part, water 1 part.

(b) ;Magnesia 12 parts, fireclay 4 parts, chrome iron ore, silica sand, furnace slag, basic steel slag 1 part each.

(c) ;Fireclay 3 parts, long fibre asbestos 1 part, sodium silicate solution 1 part.

(d) ;Carborundum 2 parts, fireclay 2 parts, sand 1 part.

(e) ;Carborundum fire sand 3 parts, fireclay 1 part, sodium silicate solution 1 part.

A good facing coat is stated to be Carborundum fire sand 6 parts, fireclay 2 parts, and sodium silicate solution 1 part.