 |  | Forming Techniques for the Self-Reliant Potter (GTZ, 1991, 194 p.) | ||
 |  | 8. Finishing and quality control | ||
|  | (introduction...) | ||
| | 8.1. Importance of finishing | ||
| | 8.2. Steps in finishing | ||
| | 8.3. Methods of finishing | ||
| | 8.4. Quality control | ||
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Finishing is the process of completing all the details on a product - since this book is about forming, we will discuss finishing only up to the stage when the product is ready for firing.
Quality control is an activity that ought to be a part of the production process, but which is very often neglected. First the producer has to decide what is an acceptable quality of the finished product and then he has to find out where in the production process control is needed to ensure that his products are up to standard. Finishing should always be followed by an inspection of quality.
Your customers will expect your products to be finished correctly, so that each piece has the same quality. It is the responsibility of the producer to guarantee his products, and to check the work at each production step.
Workers need to be instructed on their individual responsibilities for finishing, and the supervisor should be strict in rejecting products that do not meet the expected standard. The ideal system is: “Every worker is a quality controller”, but this seldom works without careful instructions and an incentive scheme.
There are several ways of paying workers:
- fixed salary: Workers are paid a fixed monthly wage, regardless of production quantity. This is not usually a satisfactory system, as workers will tend to work as little as possible.- piece rate: This was the standard system for many years in the ceramics industry. However, with the introduction of labor unions, and the idea that factory owners should provide better treatment for labor, the system is not used so much nowadays.
- basic salary plus incentive for high production: many factories now use this system. Each worker has a fixed minimum production quantity, which is the amount of work he must produce in order to pay his production costs. If he produces more than the minimum quantity, he is paid extra according to the labor cost per piece.
These systems can be introduced by considering each main step in production as a separate industry. For example, the jigger section is responsible for producing finished greenware. This greenware is inspected and counted by the supervisor, and the workers in the jigger section are paid according to the amount of acceptable greenware. Similarly, the glazing section is paid according to the amount of correctly-glazed ware, and the firing section is paid according to the percentage of successfully-fired ware.
The steps in finishing vary according to the production method. For example:
Forming on the potter’s wheel
- Before removing from the wheel: The rim must be smoothed, and the bottom is undercut to make trimming easy.
- Leather-hard: The foot ring is trimmed and smoothed with a sponge. No further finishing should be required.
- Bone-dry: The product is inspected for cracks and imperfections, and any poor quality products are thrown in the scrap container.
Slip casting
- After the casting has started to pull away from the mould, the spare line is cut.
- Leather-hard: The mould line is cut away and smoothed with a sponge.
- Bone-dry: No further finishing should be required - only inspection and rejection of poor quality products.
Joining
- After forming of the individual parts: They are joined and all excess clay from the joining, fingerprints and marks are sponged away.
- Drying: The items are turned or paired to avoid cracking and warping.
- Bone-dry: inspection of quality.
Press moulding
- After pressing: Tile edges are fettled and in the case of plastic-pressed products press seams are cut and sponged.
- Bone-dry: inspection of quality.
The methods of finishing have been discussed according to various forming systems potter’s wheel, extrusion, jigger, slip casting, etc. Here are some general hints:
- Finish ware as soon as possible. It is usually easiest to finish ware as soon as it is possible to pick it up without damage. Potter’s wheel products should be trimmed as soft as possible - waiting too long slows down the process and causes damaged products. If ware becomes too dry, it is best to throw it out, as trying to rewet it usually takes longer than starting over again. Trying to finish bone-dry ware with sandpaper is rarely satisfactory - it takes a long time, produces dust which is a health hazard, and leaves a rough surface.
- Use the correct tool. Although one tool can be used for many finishing jobs, it is best to have specific tools in order to make finishing easier. For example, special tools can be easily made for jobs that need to be repeated many times: if you need to produce several hundred 2-inch posts, then it makes good sense to make a wooden cutting box which is set up for cutting the correct length.
- Tools should be in good condition. Clay trimming tools should be sharpened regularly. Tools should be cleaned after use, and kept in a convenient location.
Acceptable quality varies according to the product. Common ware, such as low-cost storage jars, is often very roughly finished. Fine quality porcelain is rejected for even the slightest flaw, and often up to 50 % of products are not acceptable as top quality.
setting a standard
The producer must keep a constant watch on quality, and should try continuously to improve his product. A customer who receives even one batch of poor quality products is likely to look for another supplier. The level of quality depends on what quality competitors in the market are supplying and the expectation of the customers. Small tea shops will demand strong teacups, but will be more concerned about the price of the cup than its looks. So for that market quality means a cup with a strong handle, a rounded rim and no chipping or crazing of the glaze.
Quality does not mean that the products are beautiful; it means that the customer gets the product he expects. Good quality is not necessarily more costly or difficult to produce, but it pays attention to details and good management.
controlling quality
Once you have decided what your standard is for a particular product you can simply sort your finished products into two groups: acceptable and unacceptable. The rejected products could be sold at a lower price. This method is costly, because you have invested a lot of time, labor and materials in a product you will have to sell I cheap or even throw out. The most cost-effective method is to look at the whole production process and decide where mistakes could be discovered early. As soon as a mistake is discovered the product can be rejected and no more time and money are wasted on a faulty product. Let us look at an example that shows where inspection could prevent faults in the finished product (table on p. 164).
In a small industry where the producer or the supervisor is directly involved in the production he would check the production at most of the steps listed above. He could make inspection a part of another activity to save time. If the supervisor packs the biscuit kiln all products will have to go through his hands and this is a good opportunity to check all products.
In larger industries more formal quality control procedures are needed. In the example above inspection could be reduced to a check of the casting slip and a check when the products are completely dry. At that point both the quality and the quantity can be assessed as part of a piece rate pay system. The factory manager should make frequent spot checks at all steps in the production to make sure that the quality control system works.
fulfilling orders
The biggest problem for small producers is accepting more orders than they can complete in time. Rush orders are likely to be poorly finished, and customers will never accept excuses for poor quality. For that reason, producers must learn to correctly estimate production time, and should never tell customers that they can do it faster. At the time of taking orders the producers should also show their customers what quality they can expect. In ceramics production, especially in small industries, there will always be variations in color, texture, etc. The customer should be shown these and not only a few chance samples, that will be hard to reproduce.
Customers come back again if they know they can trust a producer to tell them the truth.
Slip-cast cup: quality control points
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OPERATION |
POSSIBLE FAULT |
INSPECTION |
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Slip preparation |
Pinholes |
Air bubbles in slip before casting |
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Slip casting |
Cup too thick/thin |
Casting time |
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Trimming rim |
Uneven rim, sharp edge |
Rims |
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Attaching handles |
Handles easily come off |
Dryness of cup |
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Finishing/drying |
Warping, rough surface, handles out of position, cracking |
When dry, before biscuit firing, all rejects put in clay scrap bin. |
|
Unpacking biscuit |
Cracking, overfiring |
Cracks and warping |
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Glazing |
Crawling, pinholes, running, etc. |
Dirty biscuit ware, glaze thickness |
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Firing |
Over/underfiring, etc. |
Cones, draw trials |
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Transfer to store |
All the above-mentioned standard. |
Sorting according to |
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