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Glazes should be prepared in a systematic manner in order to prevent mistakes. Most problems with glazes come from simple things, like incorrect weighing, mistakes in identifying raw materials or not sieving the glaze correctly.
Glaze mistakes are expensive, as they can result in the loss of an entire kilnload. For this reason, it is important to have the right person in charge of making glazes -cleanliness, orderliness, careful record-keeping, and reliability are required.
Most small producers do not need a large variety of glazes -in fact, many use only one or two standard glazes and achieve variety by changing the colors, doubleglazing or using engobe decoration.
Designing a glaze is somewhat like choosing a paint in the paint store. First of all, you must decide if you want a glossy or matt surface, transparent or opaque. Then you can add different colors.
Base glaze
The base glaze is simply the combination of materials that melts at the desired temperature. It is either transparent or opaque, matt, semimatt, glossy etc. without any particular colon
Glaze additions
These are usually coloring oxides that are added to the glaze. In Nepal a glaze supplying system serving small producers was established. A base glaze was supplied in 5kg bags and 8 different colors were supplied in small bags that produced standard colors when added to 5 kg base glaze. The small bags contain coloring oxides mixed with a small amount of base glaze so the colors disperse more easily in the base glaze.
Raw materials need to be as reliable as possible and always ground to the same mesh. If obtained from a glaze supplier, the materials are usually ground to at least 100 mesh. Because materials that are finely ground melt more easily, some ingredients may be as fine as 400 mesh. This is particularly true of quartz -200-mesh quartz will produce a different result than 400-mesh quartz.
When you get new raw materials, they always should be tested before using them in production. The best way is to try them in a standard glaze that you know well and to compare the results with the known glaze.
There are several steps in grinding glaze materials. Since many
of them (feldspar, quartz, limestone) come as rocks, they first need to be
reduced to pebble size. Feldspar and quartz rocks are first calcined to make
them soft enough to crush. Calcining means firing to just above 600°C. This
can be done in the cold spots of a biscuit firing or for large productions in a
special kiln. Crushing of small amounts can be done with a hammer (use eye
protection), and large amounts are usually done in a jaw crusher.
8.2.2. BALL MILLING
Ball mill operation
Ball mills are used for fine grinding of ceramic materials. The material has to be reduced to sand size (2 mm or less) before grinding in a ball mill.
Some typical uses of ball mills are:
- grinding clay that does not easily slake
- preparation of casting slips
- grinding of body additions like feldspar, quartz and glass powder
- grinding of frit granules grinding of glazes
- grinding of engobes and terra sigillata
- preparing color pigments for glaze, engobes or bodies.
There are two main types of mills:
- Large mills with an axle system are called ball mills.
- Small mills are called pot mills or jar mills.
These are usually small (up to 5 lifer) porcelain jars or plastic jars, which rotate on two rubber-covered rollers.
Conical ball mill
For large production conical ball mills are used. Various sizes of pebbles are used and the material is fed from one end and discharged at the other. Variation of the centrifugal force caused by a conical 30° slope at the discharge side classifies both pebbles and material so only fine material is discharged.
Vibro energy mill
This is a new type of grinding machine consisting of cylindric grinding chamber suspended on springs and vibrated at high frequency with the help of an excentric mounted on an electric motor. The chamber is completely packed with very hard small cylinders between which the material is filled. The vibrations make the small cylinders grind against each other and the material to be ground. The vibrating mill is better at ultrafine grinding and is more energy-efficient than ball mills.
Lining
The grinding action takes place between the pebbles, and not between the pebbles and lining. Therefore a ball mill with a steel drum can work without a lining (except for white body, where rust particles will cause discoloration). Pebbles constantly falling on a steel drum make a lot of noise. A lining will reduce the noise and at the same time prolong the life of the steel. Traditionally, linings are made of porcelain or stoneware bricks set in a cement mortar, using high alumina cement and coarse silica sand. Common cement can be used if necessary but may cause pinhole problems in glazes. The bricks should be dense and vitreous. A porcelain body for lining bricks and pebbles (fire to 1250°C or higher) is:
|
China clay |
40% |
|
Quartz |
25% |
|
Feldspar |
30% |
|
Bond clay |
5% |
One type of brick is made concave to fit the curve of the drum and another type is made for the end walls of the drum.
Linings can be made from granite, quartzite or similar hard rocks (not limestone or marble). They are cut to shape and set in a high alumina cement mortar. They last far longer than porcelain bricks. Stoneware bricks can be used for the end walls, which are worn out more slowly.
Instead of a hard lining, thick rubber sheet glued to the inside makes a very long-lasting and quiet lining.
Pebbles
Pebbles or balls can be made from vitreous clay bodies. However, it is often cheaper to collect stones of granite, quartz or quartzite along riverbeds. Flint, a variety of quartz, is excellent for pebbles. The hardness is tested with a penknife to make sure it is above 5.5 (see Mohs' scale). Pebbles of limestone are not satisfactory, as they contaminate the glaze. The shape should not be flat or elongated but spherical. (Cylinders of equal diameter and length are sometimes used to obtain particles with less variation in particle size.) Size should be between 2.5 and 5 cm in diameter.
Pebbles wear out, so occasionally take out all the pebbles for inspection. Those that are broken or flat should be discarded. In large mills, pebbles are removed when they are less than 2-3 cm. In small mills pebbles smaller than 1.5-2 cm are discarded.
Figure 8.2.2.E. A cross section of a
ball mill running at speeds from 30-90% of critical speed. At 30% the grinding
takes place mainly between pebbles and lining, at 70% a good cascading rolling
produces efficient grinding, and at 90% very little grinding takes place.
As the pebbles grind down, they contribute a small amount of material to the glaze. Usually this is not enough to make a difference in the glaze. However, if you have glaze problems that cannot be traced to any other cause, the ball mill pebbles should be checked.
Ball mill speed
Grinding of material takes place between the pebbles of the ball mill as they roll down the slope of the cylinder. If the speed is too high, the grinding action stops because centrifugal force stops the pebbles from falling.
This happens when the cylinder is running at its critical speed. Critical speed is calculated from the inside diameter of the cylinder:
(r = internal radius in meters)
The actual speed of the ball mill should be 60 -80 % of critical speed. Small ball mills can be closer to 80 % and large ones closer to 60 % Appropriate speed can be read from Fig. 8.2.2.F.
The most efficient grinding is achieved when the pebbles roll as shown in the center ball mill of Fig. 8.2.2.E. The pebbles cascade in a steady stream, and grinding takes place between the pebbles. The speed of the ball mill at 80% is too high. The pebbles have started to fall freely and this causes excessive wear as the pebbles hit one another and the lining.
Unfortunately it is not possible to look inside during milling, but if the pebbles make a low, rumbling sound the speed is correct. If they make a loud banging noise, the speed is too high or there is too much water, charge or pebbles in the mill. Porcelain jar mills crack if they run at too high a speed.
Figure 8.2.2.F. Graph of ball mill
speeds.
Charge
With a speed of 60 -80% of critical speed the charge should
be:
(by volume)
|
Pebbles: |
45 -55 % |
|
Water: |
12 -20 % |
|
Material: |
20 -25 % |
When the mill is filled to maximum capacity, the speed should be closer to 60% of critical speed. The water content should be enough to produce a thin slip. After filling, about 30 % of the volume should remain empty. If you measure all the materials separately, total volume may seem to be 85% of ball mill capacity. However, since the water and material fill the spaces between the balls, this will still result in 30% empty space.
Example
A ball mill with new lining measures inside:
width 0.64 m, diameter 0.445 m
60%-80%
of critical speed = 31.7 rpm -42.3 rpm
Charge:
(by volume)
|
Pebbles |
144-176 l. |
|
Water |
38-64 l. |
|
Material |
64-80 l. |
A typical glaze has a density (specific gravity) of approximately 2.7. That means that the glaze charge should be 24-30 kg.
Ball milling time
The time for ball milling varies with the hardness of materials. Soft materials such as frits may require only 2-3 hours, whereas hard materials like quartz can take 24 hours or more.
When you ball-mill standard materials, it is important to mill each batch for the same amount of time. For this reason, it is a wise investment to purchase a timer switch for the mill. This will avoid human errors. Too much ball milling can cause glaze crawling.
Operating procedure
Before each operation:
1) Check that the ball mill is clean inside.
2) Check that pebbles fill half of the ball mill -refill if necessary.
3) Fill in materials (20-25% of mill volume).
4) Fill water until pebbles and material are just covered.
5) Be very careful about correct ball milling time. If possible, use an automatic timer.
After operation:
6) After emptying the ball mill, clean it thoroughly with water by filling it and running it with the pebbles. If the same material is to be ground, cleaning is not needed.
Every month:
7) Empty the pebbles out and remove all pebbles that are too flat or less than 2 cm in diameter.
8) Inspect the inside lining for signs of wear, and repair as necessary.
Weighing glaze ingredients
First, you must have an accurate scale. This can be a small balance, such as is used by jewelers, or a triple beam balance, which is faster to use. Spring scales are not accurate enough, nor are postal scales. For large quantities, the most accurate low-cost balance is the common beam balance which uses standard weights.
Batch cards
For best results, a batch card system should be used. These are simply cards that have the glaze recipe written on them. As each ingredient is weighed, it is checked off on the list. When all materials are weighed, the batch card is given a number (usually the date). The same number is written on the glaze container. This makes it easier to find out the problem when the glaze does not work correctly.
Figure 8.3.0.B. Example of a glaze
batch card used for quality control.
Water
The ingredients are then added to a container which already has the approximate amount of water in it. CAUTION: The water must always be clean. After mixing, the water is adjusted. It is always best to start with less water than required. If the glaze is too fluid, it is difficult to remove excess water.
Containers
Containers for glaze should be plastic or wooden. Metal containers cause contamination and rapid settling of glazes.
Glazes are normally sieved through a 100mesh screen. The glaze should be poured through without forcing it. Never use your hand to force glaze through a sieve, as this will quickly break down the wire mesh. A brush should be used instead.
Because glazes are mixtures and not solutions, they tend to settle at the bottom of the container. Normally, the clay content of the glaze will be sufficient to keep them in suspension during application. However, some glazes tend to settle as a cement-like layer on the bottom and are difficult to stir. These glazes require the addition of a suspending agent.
Suspending agent
The most common suspending agent is bentonite, in 1-2% additions. This will normally not be enough to affect the glaze when fired. Dry bentonite cannot be added to wet glaze, as it will just form lumps and be impossible to mix in thoroughly. Instead it should either be mixed separately with water into a thin slip and then added to the glaze or it should be added to the dry glaze and mixed in well before adding water.
Binder
Another common problem is that some glazes tend to be powdery,
and come off when loading the kiln. For this problem a binder is
added.
Bentonite also works as a binder and is the simplest to use. Another
common binder is CMC gum (carboxymethyl cellulose), which is available in either
liquid or powder form. The liquid can be used directly, about 1%. The powder
needs to be dissolved in water (1:10) overnight and then is added to the glaze
as liquid.
Organic binders such as gum arable, wheat flour, sugar or starch (0.1-0.5% of dry glaze) are sometimes used. These have the disadvantage of fermenting. They should be used immediately after mixing, or if stored a few drops of chlorine bleach or formaldehyde can be added as a preservative.
Addition of 1% raw borax produces a hard surface that does not
powder when painted on.
Flocculation
Addition of a flocculation agent will make the glaze more creamy. The pottery will absorb the water more easily so glaze is picked up faster.
This works better in combination with clay or bentonite. Common flocculants are: Epsom salts (magnesium sulfate), calcium chloride, calcium nitrate and borax. They are prepared by adding 100 g flocculant to 200 ml hot water and the solution is added to the glaze one tablespoonful at a time (up to 1% of dry glaze weight). Plaster of parts (already set) can also be used.
Flocculation is also used for nonporous ware often in combination with a binder. The creamy glaze forms a thick loose layer that stays on the nonporous surface.
Deflocculation
When the glaze is deflocculated it becomes more fluid with the same amount of water. This is sometimes used for glazing nonporous ware that cannot absorb water. Sodium silicate and soda ash are the most common deflocculants and they are prepared in the same way as flocculants.
CAUTION: Binders, deflocculants or flocculants should only be added after the glaze is ball-milled.
Most potters judge the consistency of their glaze by experience and feel, or by test application to a few pieces of biscuit to see if the thickness is correct. The standard test is to check thickness with a fingernail, which is a very accurate test for an experienced glazer. Then adjust the water as necessary.
A more accurate method is to measure the specific gravity of the glaze with a hydrometer, such as is commonly used to judge the amount of water that has been mixed with milk. When reading the depth the hydrometer sinks, take care that it is really showing the correct density. If the glaze is thick you have to vibrate the bucket repeatedly to make sure the hydrometer sinks in.
Figure 8.6.0.B. Hydrometer made from
a glass test tube.
Specific gravity (s.g.) is a measure of the density of a liquid compared to water, which has a standard specific gravity of 1. Glazes will always be heavier than water. The specific gravity is found by weighing a specific volume, say 1000 ml (milliliters). If this weighs 1500 g the s.g. is 1.5. Weighing is more accurate than using a hydrometer.
After you find out the correct amount of water by trial and error, the specific gravity can be measured and future batches of the same glaze made to the same specific gravity.
CAUTION: This is not always a reliable method because the water absorption of your biscuit will vary with its firing temperature. The water will still need to be adjusted by trial and error. Trial application and testing with a fingernail still constitute the most reliable method.
If you keep wet glazes around for a long time, they will usually have problems with settling or drying up. These glazes can still be used but it will be necessary to adjust the water and to resieve them. If the glaze is extremely thick, it is sometimes best to dry it out completely, crush it and remix it.
Before using a glaze that has set in the bucket for a few days, it should always be sieved through 60 or 100 mesh.
Too much water in the glaze is also a problem. The glaze can be allowed to settle and excess water carefully taken off the top. CAUTION: With soluble glazes, this can remove some of the ingredients and result in a glaze that no longer works correctly. In this case, the water should be allowed to evaporate until the thickness is correct.
Glazes made with raw borax, or incomplete borax frits, will often grow crystals. These cannot be sieved. The glaze should be dried out, the crystals crushed and remixed.
Some glazes will develop mold and begin to smell. Although they can still be used, it is probably better to just throw them out.
The wise potter will never glaze a kilnload with untested glaze. Enough glaze should be kept on hand, so that each new batch can be test-fired in the regular glaze firing before it is used for application.
Glazes that are sold commercially are usually in dry powder form. They are made as standard glazes by ball milling, then are dried and packaged.
These glazes are simply mixed with the correct amount of water
and sieved before using.
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