There are different ways of dipping. Each way has its advocates, and there are undoubtedly good points about each. For most clays it is advisable to put a thin coat of slip on with a brush before the dipping is done. This preparatory coat should be well brushed in, and especially about the edges and ends. A flat camel's-hair brush, about the width or slightly wider than the brick, is the best for this purpose. If oil on the brick is at all bothersome, a very little strong ammonia water mixed with the slip used for brushing will be found a very good thing. With some clays that are inclined to pinhole a preparatory dip will be found advantageous. This preparatory dip may be made largely of the same material as the brick, but, of course, made very fine. I prefer, however, to use white slip, even for the preparatory dip. In case the preparatory dip is a fairly good white, the finishing slip coat need not be so thick.
For wet dipping, when no preparatory dip has been used, the slip should be as thick as it can be made and at the same time work smoothly; if ridges form from the corners diagonally across the face the slip is too thick.
There is the single-hand method and the method which uses both hands. For the first method the brick is taken by the back and, with a sweeping motion through the slip, is dipped. The brick should be held with the face to be dipped a little out of level. The edge that is in advance should be a little higher than the back edge. The edges of the brick should be vertical to the direction of the sweep; that is, if the stretcher face is being dipped, the direction of the sweep should be parallel to the heads. For bricks dipped on two contiguous sides there are two methods. One is to dip one edge, remove the brick entirely from the dipping tub and turn, and then dip the other edge. I prefer to start on the head and dip that with a curve away from the body; that is, the brick is placed in the slip with the header end below the surface and the stretcher face vertical to the surface of the slip; then, with a turn of the wrist, the brick is turned, remaining in the slip all the time, until the stretcher side becomes parallel to the surface of the slip and below its surface; then, with a sweeping motion to the left and parallel to the heads, the whole brick is removed from the slip. This second method avoids the streak near the corner that is apt to be formed by the first method.
Where two hands are used, the brick is held by each end, dipped and then turned directly away from the body. The two-hand method has the advantage of not marring the brick so much as when only one hand is used, but it is not so quick. A dipper and two boys will slip and glaze from 1,600 to 2,000 brick per day.
After the slip is on there are two courses open, either to glaze the brick while wet or to allow them to dry thoroughly before glazing. In the first case the brick are allowed to stand until the moisture from the slip has nearly all soaked into the brick and the slip has a very slight shine, something like a piece of thoroughly vitrified clay. This shine only shows in a proper light by reflection. The glaze dipping is done in the same manner as the slip dipping. If the brick are allowed to dry before the glazing is done they should be thoroughly dry, and, better, dry and hot. The face to be dipped should first be dipped Into water and then into the glaze. This method sometimes gives trouble by producing numberless small blisters. The reason for this does not seem well understood, but in my opinion it is due to the water from the glaze penetrating clear through the slip and causing an expansion in it Anyway, the dryer and hotter the brick the less apt is it to occur, and the tougher the slip used the less apt is it to occur. After the brick are finished the drying should be very slow at first, or until the slip and glaze are both well set, and the drying that is done should be with very little heat. I notice that one writer on this subject condemns steam heat at this stage of the operation. I prefer it to any other kind of heat, simply because it is easily controlled, can be turned on or off at short notice, requires little attention, and does very satisfactory work.
The arrangement of dipping room may be one of many, but, whatever it may be, it should be one of such a nature that the brick can remain upon the dipping shelves until dry unless a car system is in use. A system that requires the brick to be removed from the dipping shelves and placed elsewhere to dry adds an extra handling which tends to further mar the brick and adds to the expense of manufacture. Therefore, the dipping rooms should be combination dipping and drying rooms. This can easily be managed by having parallel rows of shelving, far enough apart to allow the dipper to work between them. Each set of shelves can be arranged with steam pipe, and the whole room can be arranged with the necessary arrangements to secure ventilation, so as to carry off the moisture. If I remember correctly, there are about ten ounces of water to be driven out of each brick, and a production of 10,000 brick per day requires the daily evaporation of 100,000 ounces, or over 6,000 pounds, or over three tons of water.
These rooms should be arranged so as to hold a day's work as nearly as possible, so that when once filled they can be closed up and not interfered with until the brick are dry.
After the brick are slipped and glazed and thoroughly dry, the surplus slip and glaze which the dipping operation has placed on the sides and ends must be removed. This is best done with a wire brush made of the old card wire from a woolen or cotton mill. This is a stiff steel-wire brush, made on leather, and can be purchased at almost any woolen or cotton mill for a mere song. The belt is usually five to six inches wide. This should be cut into pieces about four inches long and tacked on to an inch pine board a little larger than the piece, say about 4 1/2 x 6 inches. The length and weight of the wire out of which this card wire is made varies considerably. I cannot give the proper number, as I never knew it by number. The correct size is one of the medium numbers. If the wire is too coarse or too long it will chip the edges, and, if too fine, will not give good service.
With a proper brush of this character the surplus slip and glaze can easily be removed and a slight crook in the brick can be straightened. If the brush is not properly handled the corners will be rounded, edges chipped, and the brick generally be rendered anything but sightly. The brick should be placed on its edge, the one opposite to the enameled edge, the brush placed firmly on the flat side and parallel therewith, and then drawn with a sliding, and at the same time downward, motion across the brick. The motion should always be slightly away from the enameled edge and toward the body of brick, else the wires may chip off some of the slip or glaze.
If there is much tendency for either slip or glaze to peel the cleaning operation will probably make it show itself.
When dipping the brick, particles of clay are bound to drop from them into the dipping tub, and after a number of brick are dipped the slip or glaze becomes quite dirty. This necessitates constant cleaning by resieving. A very good plan is to have a sieve made of about 100 wire for slip and 120 for glaze, this sieve to be made so that it will fit into the dipping tub and be about four or five inches below the surface of the slip or glaze. By removing and cleaning this sieve occasionally all the dirt will be kept out of the tub. A small hand sieve should be furnished each dipper with which to remove particles of dirt that are seen on the surface of slip or glaze.
In setting brick on dipping shelves be careful that they are not set too far back, for, if they are, when they shrink in drying it will be found that the lower corners of many brick will be cracked loose.
After the brick are cleaned they should be directly set on the cars or other device for transporting them to the kiln. If the two-wheeled truck previously described be used, racks should be made that will fit one on top of the other, and with heavy sides a little higher than the width of a brick. Five of these racks in height can be handled by a properly constructed track, and, with thirty brick to each rack, will make a load of one hundred and fifty brick; or the racks may be made narrower, if so desired, so that the load will be reduced to one hundred brick. A wooden floor in kiln shed is not advisable, and If a brick floor is used there will be some shaking and jarring of the load in passing over it This will cause much chipping of the brick unless some precaution is taken. A very good device to obviate this difficulty is to have strips of straw-board, about one-eighth inch thick, and a little narrower than the brick, placed between each row of brick as they are placed in the racks. This will make it impossible for the brick to knock against each other, even if the load gets a severe jar.
I forgot to mention the number of dipping and drying rooms needed. It will take three days, or should take three days, to dry the brick. This, together with the day taken to dip them and the day to clean them and get the room ready for the next lot, will make in all five days, so that five such rooms will be required, and it is better to have six, so as to take just one week in making the round. Each room will take a space of 4 feet by 36 feet for each 1,000 brick daily capacity; that is, for 5,000 brick six rooms, 20 by 36, will be needed, and for 10,000 brick per day six rooms, 40 by 36 or 20 by 72, or some other proportion that will give this space. This seems like considerable space for a small capacity, but once built and paid for it will be found much more satisfactory than any attempt at concentration.
Where the whole of the dipping is done on the dry or burned brick the ordinary methods of drying may be used and the business much concentrated, but there are difficulties with dry dipping that probably more than counterbalance this advantage, and the cost of an extra setting and burning certainly is against dipping burned brick.