Most low-temperature glazes require fluxes that are either poisonous (lead) or water-soluble (sodium and potassium). Traditionally, these materials were used raw, but this is not satisfactory for modern potters. Raw lead is poisonous and sodium/potassium are water-soluble. Borax is sometimes used raw in glazes, but these glazes cannot be stored for a long time, as the borax will go into solution or form crystals.
The principle of fritmaking is very simple: molecules of poisonous or soluble fluxes should be chemically combined with glass-making materials to eliminate these undesirable characteristics.
A frit is a combination of a flux or several fluxes (lead, borax, boric acid, potassium carbonate) that is combined with other insoluble materials (quartz, feldspar, lime etc.), melted in a kiln to form an insoluble glass, and ground to be used as the base for making glazes. (Many low temperature glazes are simply 90% frit and 10% china clay).
Fritmaking is not usually practical for the small producer, as it takes time and requires a special kiln and a ball mill for grinding. On the other hand, if reliable frits are not commercially available, the potter may have to produce his own. Frit glazes are more expensive than raw glazes, but their convenience usually makes up for the additional cost.
There are many different commercially available frits, all designed for different temperatures, surface qualities, coefficients of expansion, and color responses. The potter trying to decide which frit to use must depend on the supplier, as formulas are usually kept secret. Suppliers will give advice on which frit is best for the potter's purpose. There are two main types of frit:
These are all designed to provide lead in a nontoxic form. Lead oxide is combined with other materials to give desired properties of surface, opacity, and color response. The standard lead frit is called lead bisilicate and is simply a combination of lead oxide and silica, which combines the lead in an insoluble form. This can be used as the base for a large variety of lead glazes.
Other frits used commonly in the tableware industry are called lead-borosilicate frits, which combine the desirable properties of both lead and boron and are generally safer to use.
WARNING! Lead frits can still be poisonous, and glazes made from them can be poisonous if they are not combined with sufficient silica to combine with all the lead molecules.
These are based on boron compounds, again combined with other materials. Because glazes compounded with lead are difficult to control for lead release, leadless frits are recommended for small producers.
Frit making is only suggested if reliable commercial sources are not available. Often frit manufacturers are not interested in supplying small amounts. Dishonest frit manufacturers sometimes sell bad batches of frit to small producers. Even though frit making is complicated, the small producer who makes his own frits at least has the process under his own control.
Raw borax glazes can be used, but they must be used immediately after mixing or problems will result from the soluble borax. This may be satisfactory for art pottery but, if consistent results are needed, it is better to use fritted glazes.
Similarly, raw lead glazes are widely used. This is a danger for the workers, who will eventually develop lead poisoning unless they take extreme care in handling the glaze. Modern industries never use raw lead glazes, and industrialized countries all have severe restrictions on the use of lead in glazes. In developing countries, workers in industry suffer from lead poisoning, and it is the responsibility of the industrialist alone to take care of the workers' health. As lead poisoning takes several years to develop, many factory owners do not understand the seriousness of the problem and continue to harm their workers. IT CAN TAKE UP TO 20 YEARS TO DEVELOP SYMPTOMS OF LEAD POISONING!
All the soluble materials are included in the frit batch along with silica, in order to form a glass when fired in the frit kiln. Other ma" serials may be included for modifying the frit or helping to melt it.
The main frit raw materials are:
Silica sand, SiO2
Rice husk ash, almost 95% SiO2
Borax, or sodium borate, Na2B4O7 10H2O
Boric acid, H3BO3
Feldspar, soda and/or potash, K2O & Na2O
Clay, Al2O3 2SiO2
Zinc oxide, ZnO
Red lead oxide, Pb3O4
Other materials like talc, barium carbonate and bone ash may be added.
In order to have a frit with low viscosity that easily runs out of the kiln, the clay or alumina of the glaze is not added to the frit. However, in order to make the ingredients insoluble, 2-3% kaolin should be included in the frit.
The work flow for frit production is shown in Fig. 7.2.0.A. It is better economy to prepare large frit batches when firing a continuous-type frit kiln.
Figure 7.2.0.A. Work flow of frit production.
All materials for frit need to be clean, dry and ground to pass through a 60-100-mesh sieve. The finer the material, the easier it will be to melt it. If rice husk ash is used as the source of silica, it should be well-burned to a white color, so that unnecessary carbon is not introduced. If there is a large amount of black carbon, this will decrease the amount of silica available. The content of carbon in rice husk ash may vary more than 30% from batch to batch. If materials are wet, they should be dried completely so that the weight of water is not included in the recipe. In frit calculations, the loss on ignition (see page 146) needs to be included to account for loss of material during firing.
Weigh the materials accurately and blend them together dry. WEAR A DUST MASK! Small amounts can be mixed by hand in a bucket, and larger amounts can be mixed with a shovel on a clean cement floor. After mixing the frit materials they are screened through a 16-mesh sieve (mosquito net) to ensure thorough blending or the materials are run through a hammer mill.
Melt the frit in a kiln
There are many different systems for melting frit, which are described below in section 7.2. In each system, the principle is to thoroughly melt the frit until all ingredient! are combined. Most frit is melted at 1150°C to 1250°C.
Check the frit
A sample of molten frit should be taken and examined to see if the melt is complete The frit should be uniform, without particle! of unmelted material.
With continuous frit kilns, the rate of feeding raw frit and the speed of the melted frit must be adjusted so that all the material melts completely and has time to mix' properly.
Quench the frit in cold water
The molten frit is poured into cold water, which "shatters" it into small pieces that can easily be ground. With continuous melting and discharging it is necessary to let fresh cold water run continuously.
Grind the frit
If the frit is quenched correctly, it will be easy to put it directly into a ball mill and grind it until it can be passed through a 100-mesh sieve. The granulated frit may be first dried and then stored in bags until it is needed for glaze making. Then it is ball-milled together with clay and other glaze materials. Alternatively, the still wet frit is ball-milled first.
Sieve the wet frit
When the frit is removed from the ball mill, it should be sieved through 100 mesh to remove any large particles that were not ground.
Dry the frit
The wet frit is settled, excess water is poured off, and the remaining frit can be spread out to dry, either in the sun or in a dryer.
Test the frit
Each batch of frit should be tested for correctness. The simplest way is to fire it in a kiln on a specially made flow tester, along with a sample of correct frit (page 131). If the frit flows evenly to the control sample, it will probably be correct but should be double-checked by trying it in a standard glaze.
Additionally, the frit should be tested for solubility in water. A sample amount is boiled in water for several hours, then allowed to sit for 2 weeks. If crystals do not form during this time, the frit can be considered stable. If crystals form, it means that there is not enough silica/alumina in the frit and the composition will need to be changed (frit calculations page 144). The causes of crystal formation could also be with the frit firing, e.g. overcharging, too short a firing time and improper mixing.
The finished tested frit may be sold to other ceramics producers either as a milled powder or in granular form.
There are many different kinds of frit kilns, which are selected according to the amount of frit that needs to be regularly produced.
Normally, each type of frit -transparent, opaque, lead -
requires a separate kiln to prevent contamination. When one kiln is used for
several frits, it must be cleaned out before each different batch by melting
frit in it to remove most of the old batch. This contaminated frit is then kept
separately, to be used as "clean-out" frit before changing to different
7.3.1. CRUCIBLE FRITTING
Small amounts of frit for testing are easily made in a fireclay crucible. The crucible with frit is fired together in a glaze firing, which will melt the frit into a solid block of glass. After firing, the crucible is broken away from the frit and the frit can be crushed and ground. It is a good idea to first paint the inside of the crucible with china clay slip, as this will make it easier to separate the frit. NOTE: Frits containing boric acid often cannot be melted successfully this way, as the boric acid melts at a very low temperature and flows to the bottom before the rest of the ingredients melt. Frits with rice husk ash may also be difficult to melt in this way, because the upper layer of the frit melts first sealing off the frit mixture so that the carbon remaining in the ash cannot burn out. Carbon is highly refractory and it will prevent the frit from melting.
This is only suitable for test production and is not a safe method, since the pot often cracks, resulting in frit running out, destroying other ware, kiln furniture and the kiln lining.
CAUTION: Borax frits boil during melting with a great increase
in volume. The crucible should be filled only half with frit, and a tile placed
over the top to prevent boiling over.
7.3.2. CRUCIBLE KILN
For fritting small amounts of frit a simple frit kiln is shown in Fig. 7.3.2.A. It can be fitted with several crucibles arranged in a row for melting different frits at the same time. The crucibles can be loaded with raw frit from the top. The fuel economy of this type of kiln is less than for the other kilns.
Figure 7.3.2.A. Coal-fired frit kiln with three crucibles.
7.3.3. OPEN HEARTH KILNS
Open hearth kilns consist of a tank made of firebricks, which is set in a crossdraft kiln. The kiln may be fired by coal, firewood, oil or gas. The hot flue gases heat the arch over the frit. The arch in turn heats the frit. In batch-type frit kilns, the frit melt is checked by drawing out some melted frit with an iron rod for inspection.
After the frit is completely melted, a hole at the bottom of the tank is opened and the frit flows out into cold water. Then another batch of frit may be charged from an opening in the arch.
Figure 7.3.3.A. Open-hearth frit kiln for coal firing.
The melting of several tonnes of frit may take 6-12 hours
consuming 1-1.5 tonne coal per 1 tonne melted frit.
7.3.4. CONTINUOUS FLOW
The continuous-flow frit kiln uses a kiln with a sloping floor, made of fireclay refractories. The raw frit is introduced at the upper end and, as it melts, it flows down while mixing to an exit chute by the burner and then into cold water. The kiln shown in Fig. 7.3.4.B was developed in Nepal. It uses a steam/kerosene burner, but any forced draft oil or gas burner can be used.
Figure 7.3.4.B. DSide elevation of a continuous flow kiln.
The rate of flow is controlled by introducing limited amounts of raw frit. Too much frit at one time may result in incomplete melting. If the frit runs very fast through the kiln, the low melting materials will not melt properly together with the silica. This may be a cause of water-soluble frit.
The frit can be slowed down in the kiln by making less of a
slope and by putting some obstacles in the way (like kiln shelf supports).
Rotary frit kilns are large refractory-lined cylinders, which have a burner (gas or oil) that passes through them. The raw frit is introduced, and the kiln rotates full turns (or back and forth) as the frit melts. This has the double purpose of ensuring good mixing and of transferring the heat of the firebrick lining to the frit as this constantly moves over it. When the frit is completely melted, the kiln is turned so that the frit flows out through an opening into cold water.
Figure 7.3.5.A. Front and side elevation of a rotary frit kiln. It consists of a firebrick-lined steel drum resting on rolers. It is gas-or oil-fired.
7.3.6. FUEL ECONOMY
If much frit is to be produced, fuel economy is an important factor. In general, the more frit that can be made at one time, the lower will be the fuel cost. In a continuous frit kiln, it takes several hours to heat the kiln sufficiently to melt the frit at maximum speed -this preheating period consumes a lot of fuel. It is best to fire several hundred kg of frit at the same time to reduce firing costs.
Frit industries generally use rotary kilns, as they are the most economical for long, continuous use. However, the continuous kiln developed in Nepal by the Ceramics Promotion Project compares favorably with standard fuel/frit ratios obtained with rotary furnaces.
Examples of fuel to melted frit ratios are:
Frit kiln type
kcal/kg melted frit
Open hearth coal
1 - 2 tones
6 - 12 hours
7500 - 11250
Nepal, continuous flow
1.5 - 2 tones