 |  | Forming Techniques for the Self-Reliant Potter (GTZ, 1991, 194 p.) | ||
 |  | 9. Drying of ware and moulds | ||
|  | (introduction...) | ||
| | 9.1. Drying systems | ||
| | 9.2. Drying of plaster moulds | ||
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Drying takes place when the water in the clay transfers to the surrounding air. When the air outside the clay contains as much moisture as the air inside the clay, the process of drying stops.
leather-hard
As the water leaves the spaces between the clay particles, these move closer together. That causes the clay to shrink as the water disappears; but when the particles all touch each other, little more shrinkage will take place. This stage is termed leather-hard, and there is still plenty of water left between the clay particles. The graph shows the relationship between drying shrinkage and water content of a clay.
rate of drying
Clay dries faster in warm, dry and windy weather. The rate of drying can be slowed down by covering the clay with plastic sheets or wet cloth, which causes the air surrounding the clay to become more humid.
The clay ware should dry evenly so that it shrinks evenly. A handle on a cup tends to dry faster than the cup itself, end the different rate of shrinkage will produce a crack in the handle - unless care is taken to let the whole cup dry slowly.
warping
Warping is caused by uneven drying. A tile will dry more on its upper surface so this will shrink and bend the tile upward. Cracking and warping are reduced by turning the products frequently during drying. Warping of plates and bowls is avoided by stacking them on top of each other rim to rim.
Thin clay ware dries faster and more evenly than thick ware, and is therefore less prone to cracking and warping. Thick-walled designs should be allowed to dry extremely slowly or should be made from clay with a high content of coarse materials like grog and sand.
Small products can be dried rapidly without problems. Large items require special care in drying, especially in dry weather or hot climates.
FIGURE 9-A Example of relationship
between water content, shrinkage and pores. After 50 hours the clay has become
leather-hard and shrinkage stops. The condition of the clay at points 1-3 is
illustrated in Fig. 9-B.
FIGURE 9-B Three stages of clay from
forming to dry (enlarged about 100,000 times).(1)
1) Water surrounds all particles, the clay is plastic.
FIGURE 9-B Three stages of clay from
forming to dry (enlarged about 100,000 times).(2;3)
2) The clay is leather hard. Particles touch one another.
3)
Clay is dry. Only a little water remains in the clay pores. The water remaining
between the clay particles in the pores continues to move out of the clay to the
surface, from where it evaporates. When no water remains in the pores the clay
is called bone-dry.
FIGURE 9-C Drying hints.(A; B)
A) Keep drying items away from sun and wind.
B) Rotate items
several times a day if they seem to be drying unevenly. Flatware like tiles are
sure to warp if not turned.
FIGURE 9-C Drying hints.(C)
C) Placing bowls, plates, cups, etc., rim to rim prevents warping.
FIGURE 9-C Drying hints.(D)
D) Large items should be dried very slowly, and can be loosely covered with plastic sheets.
FIGURE 9-C Drying hints.(E; F)
E) Added parts, such as large handles, can be wrapped in plastic
or painted with liquid wax emulsion, so that they do not dry faster than the
rest of the pot.
F) Items with large bottoms may crack across the center of
the bottom, because the bottom sticks to its support and when it shrinks the
periphery cannot follow. Sticking can be prevented by placing the pot on sheets
of paper or dusting its support with sand or fine grog.
FIGURE 9-C Drying hints.(G)
G) After attaching handles let the pot dry upside down. Otherwise the rim will dry faster and this may crack the handle.
FIGURE 9-C Drying hints.(H; I)
H) Typical crack of plastic-formed tiles dried in bungs. Edges
dry first, but the center of the tile is under pressure and cannot accommodate
the shrinkage of the edges so they crack. Stack tiles more openly. Dust pressed
tiles crack in the same way, but more easily because they have less green
strength and additional problems with lamination.
I) For plastic-formed
tiles: the edges dried first and the center accommodated the shrinkage. When the
center shrunk the rigid edges caused it to crack. Semidry spressed tiles: the
pressure in the center caused less stress due to differences in particle
orientation.
FIGURE 9-C Drying hints.(J)
J) Pots with handles or spouts attached should be dried slowly. Place them on the bottom of the rack where it is cooler and drying is slower. Cover with plastic if necessary.
Air can absorb water until saturated. How much it can absorb depends on how much it holds already and on its temperature. The higher the temperature, the more water can be contained in the same volume of air. There are three basic approaches to air drying of clay ware:
- A large amount of unheated air is circulated around the ware. Example: outside drying sheds for drying of bricks and roofing tiles.- A small amount of air is heated to a high temperature. This is seldom used except in the ceramics laboratory.
- A medium amount of air is heated moderately. Example: batch dryers for clay ware and plaster moulds. Rapid circulation of air is more energy-efficient for drying than high temperature.
Thin-walled crockery will dry by itself on the ware racks. Drying of heavy articles like bricks, tiles or pipes may need the help of dryers, especially in the rainy or cold seasons.
The simplest way of speeding up drying is to carry the ware outside and leave it in the sun to dry. Trolley racks make this easier.
Artificial drying is most often done by heating air and circulating it around the drying ware. For heavy clay products like sanitary ware, it is sometimes done by placing the ware on a heated floor. Infrared light is a technically good solution, but electricity is expensive. The cheapest way is to make use of the waste heat from kiln firings.
kiln drying racks
Racks for holding drying ware can be arranged around the kiln or on top of it, so that the heat radiating from the kiln during firing and cooling dries the ware. The warm kiln will create a draught of warm air that also helps drying.
drying chamber
For medium-scale industries with large outputs, or for drying plaster moulds, a drying chamber is useful. Inside the chamber a ware rack is made for ware boards, or better still, the chamber is made to hold one or more ware trolleys so that drying goods can be taken in and out quickly.
cross draught chamber
Warm air from a cooling kiln or from an additional stove is introduced from one side and a fan provides a draught across the chamber through the ware racks. A constant movement of air over the drying ware is as important as heat. The air is not saturated with water by just passing through the chamber once, so in order to economize, most of the air should be recirculated by the fan. A centrifugal fan is most appropriate for this.
The centrifugal fan can at the same time draw hot air from the kiln via a duct system. However, combustion air should not be used directly. It contains soot that will dirty the ware, along with moisture and sulfuric acids that may corrode the duct system.
tunnel dryer
The same principle can also be used in a tunnel dryer where the ware trolleys enter at one end and are taken out at the other. The trolleys should move in the same direction as the drying air.
cabinet dryer
A more simple drying chamber can be made as a cabinet with a ware board rack inside, a heating device in the bottom and a hole at the top for moisture to escape. Heating can be done by electricity or by a stove. Combustion air, except from burning gas, should not enter the drying chamber since the ware will be dirtied by soot. The hot combustion gas can instead be led through the chamber in stove pipes. Combustion itself takes place in a small stove or firebox next to the drying chamber.
mangle dryer
For large potteries a mangle dryer is a cost-effective way of drying ware and moulds. An endless chain is carrying shelves loaded with ware inside a drying chamber. The shelves are loaded at one end next to the forming unit and emptied at the other end. The size of the shelves and the speed and length of the chain are designed according to the capacity of the forming unit. This type of dryer is mainly used in jigger ware production and its initially higher cost is justified by longer mould life, lower labor cost, and saving on space, racks and ware boards.
FIGURE 9.1-C Drying chamber with a
fan (1) recirculating air and drawing waste heat through a duct (2) from the
kilns. A damper (3) regulates amount of air to be recirculated.
FIGURE 9.1-D Trolley tunnel dryer.
Waste heat enter at (A) and exits at (B).Ware trolleys enter at (C) and are
taken out at (D).
FIGURE 9.1-E Mangle dryer. The
endless chain (1) carries shelves (2) with ware and
moulds.
Plaster moulds used in casting or jigger production need to be dried regularly. Working with wet moulds slows down production and moulds that are constantly wet become soft and “rotten”. Hardness and strength of wet moulds are only half of that of dry moulds. This reduces the number of times moulds can be used, and consequently increases production cost per unit. Moulds need to be dried at least once a day if they are to last long.
cost of drying:
Drying of plaster moulds follows the same principles as for drying clay ware. However, the need for drying is more urgent, because the production requires a constant supply of dry moulds. If no artificial drying is used, it means that a large number of moulds has to be in stock. This takes up a lot of space, increases labor cost since the workers will have to carries farther for the moulds, and more money will be tied up in a large stock of moulds. All of this costs money, and with jigger or casting production of some scale, it may be more economical to use artificial drying.
recalcination of moulds:
Plaster should not be overheated, because it will then start to recalcine. Gypsum starts to calcine from around 100ºC, and since set plaster is actually gypsum, the plaster moulds also become calcined on heating. At temperatures above 50ºC, the plaster moulds will start to soften and become chalky at the edges. This will’ of course weaken the moulds and reduce their service life.
dryer operation:
When operating the dryer it is very important to make sure that the temperature inside the dryer does not exceed 50ºC. An ordinary thermometer is placed inside the dryer and its temperature is checked regularly. As long as the moulds are wet, they do not become hot even if the air is above 50ºC, but once they are dry they will suffer damage, especially on their edges and comers which dry out first. Recalcination and cracking due to thermal shock can be reduced by painting mould edges with a 10 % solution of corn sugar. It is safer to take the moulds out of the dryer when they are still slightly damp.
deflocculants:
Plaster moulds used in slip casting should be dried only from their outer surface. That means they should be kept assembled as for casting so that only a little water will evaporate from the inner mould surface. The casting slip and therefore also the water in the casting moulds contains deflocculants, and when the water evaporates, the deflocculants remain on the mould surface. Deflocculants attack plaster, so it is better if they are concentrated on the outside of the mould, where they do little harm. If the mould dries from its inner surface it will wear out faster.
dryer location:
One advantage of using a mould dryer is that less time is spent
on handling moulds. The dryer should be located close to the forming area.
Jigger moulds may be placed in the dryer with the clay items inside, so when the
moulds are taken out of the dryer they are emptied before returning to the
jigger operator. The jiggered items are then put on ware boards which are
brought to another area for finishing. Before deciding on where to place the
dryer, the work flow of the whole process should be worked out.
Additional
information on drying is given in sections 4.2.4. and
4.3.6.
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