| Following is a brief guide to pressing
candles, and thereafter are observations
on pressing disks .
The aim of this tutorial is to make clear the step-by- step ways in which purifiers are formed. However in order to really comprehend what is happening in forming candles there would be necessary a hands on experience, getting ones self involved with the clay and apparatus implicit. But it is granted that many, if not most readers will not be gaining such hands on experience, so the following is intended to give some simple idea of what's going on. The press at right uses a flywheel, part of which is shown at the top. Once the clay composition is inside the die, spinning this flywheel causes the lower end of a screw thread to apply pressure, forming a candle. Question: What pressure is needed in order to press a candle? Answer: It doesn't matter, as long as the candle is seen to be cohesive and well formed. |
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| Imagine two very different tonnages for the press, the first of which is as low as about a quarter of a ton. In the second case the pressure is high, perhaps ten tons. In the first scenario, having pressed a candle with low pressure the particles of grog and clay may not be very densely compacted, so that the purifier medium does have some small voids of air. These tiny voids do contribute to some small amount of permeability, additional to that permeability which is contributed by other factors. It should be clear that the candles formed in a low tonnage press, such as a quarter of a ton would have a little more flow by comparison to the candles formed with a ten ton press. In the situation of the higher tonnage, of course, there are a minimal number of voids between particles, which would result from pressing. So for higher tonnages the candles have a bit less flow. |
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Candles formed using presses of lower tonnage can tend to give slightly higher flow rates, by comparison to candles of higher tonnage pressing. Air voids are likely to be present, thus increasing flow. |
| Because the candles of lower tonnage presses give higher flow rates some factor may need altering, to cut the flow rate. For example, a candle formed on a low tonnage press may have within its composition a reduced amount of grog, or powdered charcoal. |
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The picture at the page header shows the die on the press, ready to be filled with the clay composition. To suggest a starting point for the would-be producer, our most recent candles are of composition 45 clay, 40 grog and 15 charcoal, all these materials 30 mesh. In the photo, left, pressure is applied to the clay composition. This is done with the help of an iron cylinder, as held in the hand of the operator, pushed into the die. If too high an average flow rate is indicated by a number of candles
of this composiiton, then a simple way of cutting back on the amount of
flow is to,
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| Water has been 15% (which is taken as a separate percentage,
with respect to the 100% of the dry clay and grog materials). After
the water is added this composition resembles a semi dry powder, and is
to be poured into the void of the die. The material is referred to
as a damp press composition, as described in the Material Processing link.
1. Prior to filling the die the composition is scooped onto a scale, to a measured amount. Recent candles have used 1.2 kgs. of the composition. Weighing helps bring about uniformity in the height of the candles, at 12.0 cms. Without the weighing, some candles would be shorter and some taller. 2. The inside of the die is coated with some kind of oil, as release, which will aid in the rmoval of the candle from the die. This release can be applied with a bruch or an old cloth. 3. The die is filled with the clay composition, after the weighing. Then this is tamped down with a stick. This tamping is a sort of pre pressing, pushing the air between particles up and out of the composition 4. Turn the flywheel, thus applying pressure to the clay composition. When the screw thread and iron cylinder are no longer descending into the die then the pressing is completed. What remains is to release the candle from the die. 5. The die, with candle inside is then removed from the stage of the press and set into a small tripod, shown in the picture below. Then this is returned to the stage of the press. The iron cylinder is then pressed further through the die, so as to extrude the candle downwards, far enough to be released. 6. After removing the candle set it to one side for drying, which generally requires a couple of days. Following the drying the candle is ready for firing. |
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A release is necessary as an aid in removing the candle from the cylindrical die. This can consist of a lubricant such as oil. In other situations a thin sheet of plastic or aluminum can be used. |
| Noteworthy is the smooth surface of the candle,
following extrusion. By contrast a candle that is not extruded, but
molded, would have a rough surface. The attributes of smoothness
or roughness need to be considered ahead of time. For example, smooth
candles can do an excellent job of arresting the particles that are contained
in water that is highly turbid. However if the water is predominantly
clear to begin with, then the rougher surfaced candles should be entirely
acceptable.
The iron cylinder can be seen entering the top of the die, under the screw. In applying this pressure the candle is to be extruded downwards. The smooth surface due to extrusion comes about when the die pushes the coarser particles into the medium, then the fine particles remain on the surface. There are several methods of bringing about a rough surface. One is to scrub the fine particles off, anytime prior to drying. Another method is to line the inside of the die with a sheet of aluminum or plastic. |
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As noted the water content should be around 15%. This is far less than the 35%, or so, that is normally used by potters, in production of most kinds of their ceramic ware. Imagine an unglazed pottery container, filled with water. While the outside of the container may get a little damp there will be practically no flow through the container. This is because as high a water content as 35 to 40% was used in mixing the clay (with respect to the 100% of the dry composition). But for the kind of plastic mass that goes along with 35% water content the clay particles pull together, thus restricting flow. And a clay film develops around the particles. The film is retained in the fired pottery container as an additional restriction of flow. |
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By contrast in minimizing water content there
is a kind of friction that pushes particles apart, thus aiding flow.
And there is no film as any further inhibition. The inference is
that reducing water content still further than 15% may be an aid to flow,
while increasing the percentage water will inhibit flow. This should
be taken into consideration as one of the variables, which when balanced
will help the producer get the desired result.
The candle can be seen coming out of the bottom of the die, into the space within the tripod. The iron cylinder is within the die. Troubleshooting |
| 1. There is associated with this type of candle pressing
a problem that is implicit to the device of the candle die. As per
the following tip, if a release is not used at the die/ clay interface
then a friction will develop on the inner wall of the die, retarding or
preventing pressing to the bottom. This is also something to know
about in case the press operator is using an inadequate release.
When this happens the resultant candle is less dense at the bottom than
it is at the top. The friction at the interface of the clay and the
die is arresting the completion of pressing. The upshot of this is
that the top of the candle gives less flow than the bottom of the candle.
As a result the candle is a reject, since it's inconsistent flow rate cannot
be further assessed.
2. In case the smooth surface is not desirable, the water to be filtered not turbid, then the fine particles of this surface may be removed. It is, however, possible to prevent the smooth surface developing. In this situation a sheet of aluminum or plastic can be imposed between the clay and the die. A release consisting of oil, can be used, between the aluminum and the die. And no release is used between the clay and the aluminum. Then the extruded candle has a sheet of aluminum around it. After this sheet is removed the candle will have less fine material at the surface. But as is the situation with #1., it is necessary to ascertain the oil, or other release that works best. One approach is to use excessive oil in the beginning, cutting back on the amount of this in subsequent pressings. By comparing results with different amounts of release an optimal application can be determined. |
| About sealing disks into the pottery containers, this is a simple matter,
using white cement. This cement is of the same type recommended for
sealing the candles into plastic cover caps. But for sealing the
filter disks into plastic containers the right sealant needs to be found,
in whatever local market. This may require a bit of experimentation,
trial and error. Also important is that the sealant should penetrate
into the porous pottery medium. If the sealant does not penetrate
then its interface with the pottery may provide a pathway for bacteria
to pass quickly into the filtered water.
A disadvantage of pottery containers is that their porous, inner surfaces can be breeding grounds for bacteria. So their frequent cleaning is required. Plastic containers do not have this disadvantage, requiring far less frequent cleaning. But despite this disadvantage of porous pottery surfaces it may well be considered desirable to offer pottery containers as an alternative. This is especially true because it is possible to disinfect the porous surface by coating this with colloidal silver, or performing other silver treatment. Another alternative for silver treatment of a pottery container would be to paint the inner surface with silver nitrate and allow to dry. Then fill the container with salt water, overnight. After draining the salt water fill the container several times with non salty water. This should leach away the remaining salts that are within the filter wall. Conclusion Please address inquiries and observations to:
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