The clay as brought in is slightly roasted (Note 1) in heaps under large sheds, each heap when finished representing one month's production. The stone is delivered into the works in wagons containing about ten tons, the contents of each are sampled, and the carbonate of lime determined before unloading. The clay is free from lime, consequently the composition of the stone being known a simple calculation gives the proportion of each material. The mixture is by weight and is finely crushed in a swing jaw crusher and elevated to hoppers above the grinding mills. The grinding is in two stages. In the first, a somewhat coarse gritty powder is obtained by grinding between burr stones. This is finished by tube mill to such a fineness that less than five percent of residue is left upon a sieve having 40,000 meshes to the square inch (Note 2). At this stage the composition of the mix is checked, the percentage of carbonate of lime being determined in samples taken at intervals. By the method employed a determination, accurate to with one half of one percent, can be made in seven minutes. The raw meal or flour as is it technically called is received into a series of tall receptacles or silos which not only serve as storage, but also as mixers. By means of extracting worms a constant stream of previously ground material is drawn out and is returned with the stream coming from the mills, while the other worms are employed to extract that required for the next step of the process (Note 3). The material extracted is collected into one conveyor at the delivery end of which it is mixed with about ten percent water. The damp mixture is conveyed to a floor above the presses, and by these it is formed into bricks which are passed directly to the kilns, of which two types are at present in use. An older one which works intermittently, and for which a portion of the succeeding charge is dried in chambers (Note 4) by heated air obtained during the burning of the preceding one, and one shaft kiln just set to work (Note 5). This works continuously, bricks as taken from the presses and coke being continually charged at the top, while clinker is drawn below. The kiln itself is constructed of steel plates lined with fire bricks and it works with natural draught, special arrangements having been made to ensure sufficient air supply, and consequently rapid burning and cooling of the charge. The charging floor is carried on an enclosing structure built up of broad flange beams, which also supports the chimney quite independent of the kiln. This is lined with specially made thin fire bricks built in cement and the plates are bolted (?) together. A scaffold formed of light (?) quartering was carried up with each belt of plates and firmly clasped to them. This at once served for lifting and bolting the plates and for putting in the lining. This type of kiln is repeated with some slight modifications. The whole is driven by a Pollit and Wigzell compound condensing engine of 450HP. The lighting is by arc lamps, power for these being obtained from a separate engine. An inspection of the laboratory bought the visit to an end.
Vote of thanks from Mr Platt of Salford to Mr Spackman of Isis:- "it was part of his duty to test cement for the large Corporation of Salford and in testing cement he had a pleasant experience a few years ago. Two samples of Portland cement were sent to him, one being highly priced and one being much cheaper. The price of the latter was so low indeed that the
Committee who had advertised for contracts were very doubtful about it. The Committee sent samples to him and he did not know where either of them came from. The samples were both tested in the same way and the cheaper cement fulfilled the standard to a greater extent than the cement of a higher price. He afterwards found out that the cheaper cement was manufactured by the Isis Portland Cement company, of which Mr Spackman was the head."