The fine grinding of cement clinker is distinctively Edisonian in both origin and application. As has been already intimated, its author followed a thorough course of reading on the subject long before reaching the actual projection or installation of a plant, and he had found all authorities to agree on one important point—namely, that the value of cement depends upon the fineness to which it is ground. [16] He also ascertained that in the trade the standard of fineness was that 75 per cent. of the whole mass would pass through a 200-mesh screen. Having made some improvements in his grinding and screening apparatus, and believing that in the future engineers, builders, and contractors would eventually require a higher degree of fineness, he determined, in advance of manufacturing, to raise the standard ten points, so that at least 85 per cent. of his product should pass through a 200-mesh screen. This was a bold step to be taken by a new-comer, but his judgment, backed by a full confidence in ability to live up to this standard, has been fully justified in its continued maintenance, despite the early incredulity of older manufacturers as to the possibility of attaining such a high degree of fineness.
[Footnote 16: For a proper understanding and full
appreciation of the importance of fine grinding, it may be
explained that Portland cement (as manufactured in the
Lehigh Valley) is made from what is commonly spoken of as
"cement rock," with the addition of sufficient limestone to
give the necessary amount of lime. The rock is broken down
and then ground to a fineness of 80 to 90 per cent. through
a 200-mesh screen. This ground material passes through kilns
and comes out in "clinker." This is ground and that part of
this finely ground clinker that will pass a 200-mesh screen
is cement; the residue is still clinker. These coarse
particles, or clinkers, absorb water very slowly, are
practically inert, and have very feeble cementing
properties. The residue on a 200-mesh screen is useless.]
If Edison measured his happiness, as men often do, by merely commercial or pecuniary rewards of success, it would seem almost redundant to state that he has continued to manifest an intense interest in the cement plant. Ordinarily, his interest as an inventor wanes in proportion to the approach to mere commercialism—in other words, the keenness of his pleasure is in overcoming difficulties rather than the mere piling up of a bank account. He is entirely sensible of the advantages arising from a good balance at the banker's, but that has not been the goal of his ambition. Hence, although his cement enterprise reached the commercial stage a long time ago, he has been firmly convinced of his own ability to devise still further improvements and economical processes of greater or less fundamental importance, and has, therefore, made a constant study of the problem as a whole and in all its parts. By means of frequent reports, aided by his remarkable memory, he keeps in as close touch with the plant as if he were there in person every day, and is thus enabled to suggest improvement in any particular detail. The engineering force has a great respect for the accuracy of his knowledge of every part of the plant, for he remembers the dimensions and details of each item of machinery, sometimes to the discomfiture of those who are around it every day.
A noteworthy instance of Edison's memory occurred in connection with this cement plant. Some years ago, as its installation was nearing completion, he went up to look it over and satisfy himself as to what needed to be done. On the arrival of the train at 10.40 in the morning, he went to the mill, and, with Mr. Mason, the general superintendent, started at the crusher at one end, and examined every detail all the way through to the packing-house at the other end. He made neither notes nor memoranda, but the examination required all the day, which happened to be a Saturday. He took a train for home at 5.30 in the afternoon, and on arriving at his residence at Orange, got out some note-books and began to write entirely from memory each item consecutively. He continued at this task all through Saturday night, and worked steadily on until Sunday afternoon, when he completed a list of nearly six hundred items. The nature of this feat is more appreciable from the fact that a large number of changes included all the figures of new dimensions he had decided upon for some of the machinery throughout the plant.
As the reader may have a natural curiosity to learn whether or not the list so made was practical, it may be stated that it was copied and sent up to the general superintendent with instructions to make the modifications suggested, and report by numbers as they were attended to. This was faithfully done, all the changes being made before the plant was put into operation. Subsequent experience has amply proven the value of Edison's prescience at this time.
Although Edison's achievements in the way of improved processes and machinery have already made a deep impression in the cement industry, it is probable that this impression will become still more profoundly stamped upon it in the near future with the exploitation of his "Poured Cement House." The broad problem which he set himself was to provide handsome and practically indestructible detached houses, which could be taken by wage-earners at very moderate monthly rentals. He turned this question over in his mind for several years, and arrived at the conclusion that a house cast in one piece would be the answer. To produce such a house involved the overcoming of many engineering and other technical difficulties. These he attacked vigorously and disposed of patiently one by one.
In this connection a short anecdote may be quoted from Edison as indicative of one of the influences turning his thoughts in this direction. In the story of the ore-milling work, it has been noted that the plant was shut down owing to the competition of the cheap ore from the Mesaba Range. Edison says: "When I shut down, the insurance companies cancelled my insurance. I asked the reason why. 'Oh,' they said, 'this thing is a failure. The moral risk is too great.' 'All right; I am glad to hear it. I will now construct buildings that won't have any moral risk.' I determined to go into the Portland cement business. I organized a company and started cement-works which have now been running successfully for several years. I had so perfected the machinery in trying to get my ore costs down that the making of cheap cement was an easy matter to me. I built these works entirely of concrete and steel, so that there is not a wagon-load of lumber in them; and so that the insurance companies would not have any possibility of having any 'moral risk.' Since that time I have put up numerous factory buildings all of steel and concrete, without any combustible whatever about them—to avoid this 'moral risk.' I am carrying further the application of this idea in building private houses for poor people, in which there will be no 'moral risk' at all—nothing whatever to burn, not even by lightning."
As a casting necessitates a mold, together with a mixture sufficiently fluid in its nature to fill all the interstices completely, Edison devoted much attention to an extensive series of experiments for producing a free-flowing combination of necessary materials. His proposition was against all precedent. All expert testimony pointed to the fact that a mixture of concrete (cement, sand, crushed stone, and water) could not be made to flow freely to the smallest parts of an intricate set of molds; that the heavy parts of the mixture could not be held in suspension, but would separate out by gravity and make an unevenly balanced structure; that the surface would be full of imperfections, etc.
Undeterred by the unanimity of adverse opinions, however, he pursued his investigations with the thorough minuteness that characterizes all his laboratory work, and in due time produced a mixture which on elaborate test overcame all objections and answered the complex requirements perfectly, including the making of a surface smooth, even, and entirely waterproof. All the other engineering problems have received study in like manner, and have been overcome, until at the present writing the whole question is practically solved and has been reduced to actual practice. The Edison poured or cast cement house may be reckoned as a reality.
The general scheme, briefly outlined, is to prepare a model and plans of the house to be cast, and then to design a set of molds in sections of convenient size. When all is ready, these molds, which are of cast iron with smooth interior surfaces, are taken to the place where the house is to be erected. Here there has been provided a solid concrete cellar floor, technically called "footing." The molds are then locked together so that they rest on this footing. Hundreds of pieces are necessary for the complete set. When they have been completely assembled, there will be a hollow space in the interior, representing the shape of the house. Reinforcing rods are also placed in the molds, to be left behind in the finished house.