Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.) 14. Quality control (introduction...) 14.1. Raw materials control 14.2. Glaze preparation control

Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.)

14. Quality control

A successful business depends on consistent results. This can only be done if quality control is made a habit. This means having regular procedures for storing glaze materials, checking new shipments, weighing, grinding; mixing, and checking each new e batch of glaze before using it in production.

14.1. Raw materials control

Raw materials suppliers have their own problems with getting consistent materials. Sometimes they may send you a different material without any notification, or the quality of material from the mine may change. If you have enough working capital and storage area, it is best to get raw materials in large quantities, up to one year's need.

14.1.1. RAW MATERIAL TESTING

When you get a new shipment of raw materials, each one should be tested. For the individual potter this is simply done by testing each one in the standard glaze recipe to see if there is any change. About 200 g glaze is mixed using the old stock materials and replacing only one of the new materials at a time. If the test glaze is different from your standard glaze, it will be necessary to alter your glaze recipe.

14.1.2. STORING OF GLAZE MATERIALS

All materials should be kept in bags or buckets so there is no chance of mixing up different materials. Mark the contents of all bags and buckets and the material's delivery date on labels that cannot easily be removed. Keep the glaze material store separated from working areas and make sure that only responsible persons have access to it.

14.2. Glaze preparation control

Many glaze problems are caused by carelessness during mixing of the glaze. When preparing glazes and frit, make sure that the right recipe is used and that the weighing is done correctly.

Batch cards

If you are running a small pottery and you are doing all glaze work yourself, you can rely on a very simple system. Still, write down your recipe, keep it next to the balance and after weighing each material tick it off on the recipe.

For larger productions use a batch card system. A batch card form is shown in Fig. 8.3.0.B. The card follows the glaze batch during its preparation and later when the glaze is used in production. It has three purposes:

- It shows the glaze mixer the recipe, ball milling time, density of the glaze slip.
- The supervisor can easily check if all instructions are followed.
- If something goes wrong, the batch card helps to trace the cause of the problem.

The batch card number should be marked on the glaze bucket. To avoid mistakes tie a tile glazed with the same glaze to the bucket.

Balance

The balance and the weights need to be checked now and then. The weights should be clean. The balance may become inaccurate because the scales get dirty or the pivots or beams get out of alignment. After cleaning the weights they and the balance are checked by weighing something with a known weight ( 1 lifer of water weighs 1 kg).

Graduated cylinder

Cylinders or flasks used for measuring volume are used for adjusting density of glaze slips. Unfortunately, measuring cylinders are often not graduated correctly by the manufacturer. The cylinder can be checked by filling it with water to its mark, say 250 ml and then checking if the water weighs 250g. In some cases they have been out by more than 10%.

Ball milling

The fineness of the glaze particles influences the glaze very much. To keep this constant, make sure that the ball milling time is the same. The time should be noted on the batch card. If different glazes are milled in the same ball mill, the worker must enter on the card that he has cleaned the ball mill before loading it. The supervisor should check that the ball mill lining and pebbles are correct.

Sieving

The glaze should be screened before use. On the batch card screen mesh size is mentioned. Check the residue on the screen. If you get more residue than usual, there may be something wrong with the ball milling.

14.2.1. METHODS OF TESTING BATCHES OF GLAZE AND FRITS

Testing frit

Molten frit can be drawn from the frit kiln to see whether all ingredients are well melted and whether air bubbles are released. Air bubbles may not be a problem, since many of them will be released during grinding and the second glaze firing. But if air bubbles (pinholes) give trouble during glaze firing, it may be a good idea to extend fritting time, so that the air has time to escape. In continuous frit kilns, bars of refractory brick can be placed on the sloping floor to slow down the flow of frit.

After fritting is over, the melting temperature and the viscosity of the frit can be compared with previous batches of frit by melting a fixed amount of frit on a sloped tile.

Testing glaze

Each new batch of glaze should be made at least one firing before using it. This will give enough time to apply the glaze to a few test pieces and fire them in the regular glaze firing. Glaze at least three pieces and place one in a cold spot, one in a normal and one in a hot spot. If something is wrong with the glaze, this will prevent a whole kilnload from being ruined.