While the characteristics of the stones and the manner in which they were used and maintained evolved over time, the same basic process never changed. In the mid 19th Century stones generally came from France, and were cut and dressed to have grooves in them, so that they both gently cracked the wheat and ground bits and pieces that mixed into the flour. In addition to flour purity, other considerations included not heating the flour while it was being ground. In general the process of creating and maintaining a proper face on the stone — known as "dressing" the stone — was more of an art than a science. Stevens' partner Tom Oborn had a method to dress the stones that was non-traditional, but developed a higher yield. One part of dressing the stone consisted of "picking" whereby typically the faces were chipped and roughened slightly. Oborn instead left the surfaces in a smooth state, but otherwise adjusted the width of the stones and their speed to compensate. Stevens surmised that Oborn's process did less crushing of the outer shell, but instead gently cracked the berry open and dumped out the endosperm. It was from this analysis that Stevens arrived at his invention idea. Between 1870 and 1872, Stevens tried a number of models, including wooden rollers, but became convinced that rollers made from steel would best provide the type of cracking necessary. In order to prove the point, Stevens needed an actual device and he had some difficulty finding a machine shop who could manufacture steel rolls that had sufficient length and diameter, eventually employing the firm of Farrell and Sons in Ansonia, Connecticut. Once the rolls were made, Stevens developed systems to feed the wheat into the rolls, and he optimized the yield by experimenting with speeds and roller-to-roller distances. In time he was able to improve the yield of the mill from 200 barrels a day using the stone process, to 500 barrels a day using the steel roller process, with the same amount of water power. In addition to this yield per horsepower, the flour yield per bushel of raw wheat also increased. Almost immediately the mill began benefiting financially from the new process, which Stevens and his associates attempted to keep secret but which was copied by rivals nevertheless, as they learned about it through various subterfuge. In order to protect his rights to the invention, in 1877 Stevens applied for a patent, and in 1880 was awarded . The claims of this patent were:  These claims covered most of the important aspects of the invention (differential speed, plane surfaces at fixed distance to optimally crack the berry, crossed grooves), but was not sufficient to protect all he had done, so subsequently, Stevens applied for several other additional patents, to cover the aspect of continuous reduction (re-crushing of the bran) and separation.