Three weeks later, 13/02/1891 p126, Blount suggests in a roundabout way that Stokes' cement is unsound, while scoring one-upmanship points by citing a French journal:
STOKES' PORTLAND CEMENT PLANT.
SIR, - Remarking parenthetically, in reply to Mr. Wilfrid Stokes' remarks as to the completeness of the combination of the constituents of Portland cement produced by his process, that the distances between adjacent particles of chalk and clay even in well washed slurry are, molecularly speaking, colossal, I will, with your permission, narrow the question to the highly practical point - is the cement produced of really high quality? Mr. Stokes has put forward evidence that it is, and until quite recently the tests upon which he relies have been the best available. Specialists in Portland cement are well aware that the most conclusive of all tests are those extending over one, two, or even more years, when latent defects which are overlooked by the ordinary short-time tests often become apparent. In particular, the presence of a small proportion of free lime has been hitherto practically impossible to detect, but would nevertheless make itself felt in the long run. Most fortunately, we have now for the first time a means of ascertaining with considerable certainty, and in a short time, the quality of cement which could only be pronounced upon by older methods after the lapse of a period impracticably long.
In the Bulletin of the Société d'Encouragement pour l'Industrie Nationale M. Deval has elaborated a method of hot testing , which has proved in his hands capable of nicely discriminating between good and indifferent cements, and which will, doubtless, speedily find a place in the specifications of engineers using large quantities of Portland cement.
Had this method been generally known a short time back, Mr. Stokes would have been able to advance still more conclusive proofs of the good quality of the cement made by his process. I may say that I am so impressed with its utility, both for such experimental purposes and for supplementing the ordinary routine of cement testing, that I am taking steps to introduce it into our cement laboratory here.
BERTRAM BLOUNT. Laboratory, Broadway, Westminster, S.W., February 2nd.
On the same page, Stokes responds to the attack on the originality of his patent:
SIR, - I cannot allow Mr. Smith's remarks on my process to pass without correction, but as his information appears very incomplete, he will no doubt be glad to be set right. Mr. Thwaite's patent – the complete specification of which is dated May 19th, 1888, and which I presume is the one referred to - has no resemblance whatever to my own, except in so far as that revolving furnaces are proposed to be used in both. The gist of the proposal is to divide the burning into two stages in separate furnaces, leaving the preparation of the slurry to "a machine suitable for the purpose," and the cooling of the clinker to chance; the heating of the air being effected by regenerating chambers, &c. As regards revolving furnaces for cement burning, Mr. Smith again appears misinformed. At the present time new works are almost completed near Grays, where six revolving furnaces are to be used, their adoption being considered justified by the experience obtained with Ransome's furnace at Messrs. Gibbs' works. This does not look like the revolving furnace being a failure for cement burning. It is true that although upwards of 900 tons of cement have been burnt by Ransome's process, it has not been altogether a success, and for three very good reasons: (1) The endeavour to burn as a powder; (2) the absence of any attempt to prepare the slurry for burning; (3) the lack of means for cooling the clinker.
In my apparatus, however, those difficulties are overcome, as the slurry is automatically dried and delivered to the furnace in small sticks, which gets over the powder difficulties; and the burnt clinker-cooling cylinder not only cools the clinker, but also does away with a regenerative system of flues, which were a constant source of trouble to Mr. Ransome. With these wide differences from Ransome's process, I fail to see how Mr. Smith arrives at his conclusion that mine "would fail in the same way and from the same cause as Ransome's."
Just one word of warning to any of your readers who may feel tempted to join Mr. Smith and "the well known specialist" in a trial of the new "revolving calciner." A furnace pure and simple won't do. The apparatus must deal with the slurry as it leaves the pumps, and must carry on the operations of drying, burning, and cooling continuously, and without hand labour, otherwise a thorough commercial success cannot be obtained.
9, Victoria-street, S.W., WILFRID STOKES. February 3rd.
Also on the same page, a new critic puts forward the general view common among those with static kilns, that making clinker takes a long time. Modern rotary kilns complete the whole process in 15-20 minutes. A mention of a six-day working week hints at another concern of the many manufacturers who still insisted on shutting their plant down on Sundays.
SIR, - In your issue of 30th ult. Mr. Stokes - in reply to the letter by Mr. Blount, which appeared in your issue of 23rd ult. - states that the short time required to produce a good cement clinker by his continuous process is principally based on the assumption that the slurry is properly washed, but admits that in the case of badly-ground slurry, time is distinctly an element in the proper burning of cement, as the larger particles of chalk then take a considerable time to soak up, so to speak, the silicate of alumina.
Now, all cement manufacturers being aware of the absolute necessity of washing their slurry to the finest degree, generally provide that the slurry shall pass through a fine sifter of an approved number of meshes, so that as a rule, it is of an uniform consistency, and any "larger particles" are not allowed to leave the washmill until thoroughly dissolved, and any attempt to produce cement from slurry improperly washed would only prove disastrous in every way.
What, then, would Mr. Stokes define as properly washed slurry? for thus far his explanation is applicable to all processes of burning good cement clinker.
It would, no doubt, be interesting to many of your readers to know whether any other method superior to the washmill is adopted by Mr. Stokes for obtaining a more perfectly ground slurry than the washmill or other known methods produce.
Mr. Stokes has succeeded in obtaining satisfactory results from his apparatus on a small scale only, but if he could give some idea of the quantity of clinker, approximately, which his apparatus would be capable of turning out - say, during six working days with the amount of coal-gas likely to be consumed, your readers would be better able to form a more definite idea as to the value of such a process, as against kiln-burnt or other existing methods.
The time which Mr. Stokes allows - only ten to fifteen minutes for the thorough calcining of the slurry into cement clinker, is so very short, as compared with other methods, that this fact alone, to say nothing of the great saving in fuel effected, would, if practicable on a large scale, want no other recommendation for its adoption by all large cement manufacturers. From numerous and exhaustive tests and experiments, during several years' experience as a manufacturer of cement, with a view to determining the minimum length of time necessary to properly burn and produce good clinker from the dried slurry I regret to say that under the most favourable circumstances I have never succeeded in bringing the minimum of time under two and a-half hours and this at a temperature of about 1600 deg. Centigrade, and any attempt to hasten this process by the introduction of additional heat only proved useless, the result being that the outside crust of the slurry is simply burnt to a clinker as hard as, and similar in appearance to, the ordinary coal clinker formed on furnace bars, whilst the inside being partially protected by this coating remains almost in the same raw state in which it first entered the kiln.
PORTLAND, 20, Falsgrave-road, Scarborough, February 9th.