Having pointed out the necessity of care in the mixing of the material, we will now describe the apparatus used from time to time in the process. Where white lead grinding in oil is carried on a large scale, as is done by corroders, the building in which the operation is going on is usually so arranged that the dry lead is stored in hoppers or bins on the top floor, where it may be shoveled into the mixers that are let into the floor, and where supply tanks or pipes are at hand to furnish the necessary oil. These mixers are provided with the necessary shafts and blades to bring about the required consistency of the paste, and may be of more or less depth, reaching to a convenient height above the floor below, where stone mills with large hoppers are ready to receive the mixing that is passed from a gate that may be on the side or on bottom of the mixer. These mixers may be of any length, and in some instances have been constructed of a depth of ten or even twelve feet, but in the opinion of the writer are really impractical because they cannot be readily cleaned.
The most practical mixers are of a diameter of about 36 to 40 inches and not over 30 to 36 inches in depth, and they should be twin mixers, i. e., so arranged that the mill hopper can be fed from either of the two. This will give ample time to mix lead and oil in one mixer, while the material is discharged from the other, and there is no occasion to unduly hasten the mixing, for it goes without saying that thorough mixing assists the process of grinding to a great extent, as otherwise the mill is compelled to do one-half of the work that should be done by the mixer. The favorite mills for white lead grinding have always been stone mills of 36 inches diameter, although in some cases the diameter preferred was 42 inches, the stones being of the French buhr type, because it was the belief that these did not only the best and finest grinding, but also imparted a polish to the finished product. While these buhr stones last a lifetime in lead grinding, it is a fallacy to think that they do better work than the so-called esopus stones, and it has been proved that the mills provided with the latter show a much larger output on all pigments of soft texture than the buhr stones. The principal advantage, after all, in milling white lead is the arrangement of the speed of the mills to correspond to the diameter of the mill stones or grinding surface and the proper dress of the stones, as well as the diameter of the eye of hopper, which really controls the feed. This should not be smaller than 10 inches for lead in oil of stout consistency, yet for a long time 5 or 6 inches diameter was thought to be the proper opening for the hopper of a 30 or 36 inch mill.
The speed of a 36-inch white lead mill should not exceed twenty-six revolutions per minute, otherwise the product will be overheated, as it will be at any rate, if the attendant is not careful to watch it. The idea of paint grinders always has been that unless the mill stones become hot there can be no fine grinding, and some have tried to help it along by heating the oil in cold weather, which is also a fallacy and liable to lead to erroneous conclusions. A stone mill soon becomes hot when tightened up well or set close, and therefore it behooves the miller to take off a turn or two as the heating makes itself apparent. Unless he attends to this he is liable to ruin the mill stones as well as his product. When white lead in oil as it comes from the mill indicates a temperature of over 160o F., it is not a good product, as before stated, in point of covering power. This is why some white lead manufacturers have abandoned the use of twin mills and substituted chasers and roller mills.
This apparatus is best arranged with the chaser located so as to discharge the lead and oil after being well condensed into a strong mixer that is set into the floor with its body directly over a roller mill with granite stone rolls, the outlet or gate of mixer discharging on the center of the back roller. The idea of this style of white lead grinding is to crush the dry lead by the heavy weight roller in the bed of the chaser, adding as much oil as is required to produce a stiff paste, not unlike heavy putty in consistency, which imparts increased density to the product, but in order to overcome any inclination of the material to be gummy, it is run into the intermediate mixer and more oil is added to form a good medium paste. When the mixing is finished, i. e., when lead and oil are one uniform mass, the gate of mixer is opened, the rollers started and the paste lead given its polish by the rubbing and grinding of the rollers. As the lead in oil ground by this process does not become overheated, as is often the case on stone mills, it may be put directly in the packages required for commercial purposes. In some factories steel rollers have been preferred over the stone rollers, but the writer prefers the latter as least liable to discolor white pigments. If the dry white lead is soft and free from horny particles, the chaser, mixer and roller process of milling will produce the best stiff lead for flat work, because the percentage of oil required is less on account of the condensing action of the chaser, and the consumer does not have the trouble of "drawing" the oil for flat work in jobs of decorating. When from 9 to 9 1/2 pounds of oil is required for 90 1/2 to 91 pounds of dry lead for each 100 pounds of the paste in stone mill grinding, 8 to 8 1/2 pounds of oil for 913 1/2 to 92 pounds of dry lead will produce a paste of similar consistency when using the chaser, mixer and roller mill. The power cost will be a little more, but the lesser cost for labor per ton will even this up.
As noted above, in grinding on stone mills the lead in oil is discharged rather hot, especially during the warm season, and it cannot be placed directly into commercial packages without the risk of forming a lump in center of container on cooling, or a crust on the side of the package; in short, the lead should be cooled to a normal temperature, which is best accomplished by a mechanical stirring device, a so-called cooler that is running at same speed as the mill and is in the shape of a shallow mixer, with mixer blade, from which the lead may be put up in containers same as from the roller mills. Years ago the mills were set fairly high above the milling floor and each set of mills was provided with a water-cooled turntable in front on which the lead in oil dropped from the mill scraper and was then scraped off in turn by a scraper attached to the table. This worked very well during cool seasons, but was of little or no use during the heated term.