Slip cast bonsai pots
The slip cast technique is an important method for the production of bonsai pots. It is mainly used for the mass production of bonsai pots for indoor bonsai. Usually only smaller pots up to 30cm are produced with this technique.
Casting clay technology - step by step
The following work steps are involved in casting clay technology:
- Production of the master mold and a large number of working forms. Many hundreds of identical work molds are often used in production.
- Procurement of the clay. Usually this is bought. Sometimes it is made from clay soil in the pottery (grind clay in a drum mill ➔ sieving ➔ dissolve in water with a stirrer ➔ press into bars or plates ➔ dissolve in water again before using and sieve again).
- Working molds are filled with casting clay up to the upper edge. After a few minutes, a little more liquid clay has to be poured into the working mold, as the plaster mold draws water out of the clay and thus the volume of the casting clay in the mold is reduced.
- As the dry plaster mold draws water out of the liquid clay, a thicker layer of clay forms on the wall of the plaster mold over time. If this layer is thick enough for the pot wall, the remaining liquid clay can be poured off after a while (approx. 30-60 minutes).
- The plaster molds on the heated tables continue to draw water from the clay layer. In the next few hours this makes it leather-hard and stable enough to remove the pot blank from the mold.
- The bonsai pot blank is removed from the mold and further dried on the heated tables. It must be ensured that the bottom of the pot does not warp when drying. It is best to support it with some clay.
- If the blank is stable enough for post-processing, unevenness can be smoothed and burrs removed. This step is very important for the quality of the future bonsai pot. Now the blank is flexible enough to compensate for bulges (e.g. an uneven floor).
- Now the pot is dried further. If it is stable and dry enough, it can be fired immediately (unglazed bonsai pots) or after the glaze (usually dipping or possibly spraying), usually in a tunnel oven.
- Firing in a clay kiln (mostly tunnel oven): These kilns are usually 30-50m long. On the one hand, the pot blanks are often placed on fireclay plates and slowly pushed through the furnace with a hydraulic press. The firing process takes place in the middle of the furnace. I.e. up to the middle the pot blanks warm up slowly to the firing temperature, are fired in the middle and slowly cool down again towards the end of the tunnel kiln. The entire firing of a bonsai pot takes so many hours. Typically around 10-12 hours, depending on the length of the tunnel kiln and the feed speed.
- The firing in a tunnel kiln is a continuous process. This means that once a kiln has been heated, it will be continuously charged with pottery to be fired for several months, 24 hours a day. The kiln wears out over the months. Above all, the fireclay insulation suffers so that the stove is worn out after about 6-12 months. Then the workers let it cool down and it is usually completely replaced by a new kiln.
- Tunnel kilns often have several channels that can be loaded with pottery in parallel. This means that many thousands of bonsai pots can be fired per day with a multi-channel tunnel oven. There are potteries that produce several million bonsai pots and coasters per year with such tunnel kilns.
- After firing, quality control, packaging and delivery to the customer take place. The main market for bonsai pots produced in chinese potteries using this method is China itself. In individual regions of china, many millions of bonsai are cultivated on hundreds of square kilometers, all of which need a bonsai pot. And these numbers are underestimated rather than exaggerated.
Slip cast technology - pictures
The following pictures (compiled from several potteries) give a small impression of the work steps:
Advantages and disadvantages of the slip cast clay technique
- A decisive advantage of the slip cast clay technique for the production of bonsai pots is the low production costs. The clay does not have to be laboriously pressed into a plaster mold. As the name suggests, it is poured into a plaster mold. This is of course much faster.
- Due to the low production costs, these pots (mostly glazed bonsai pots for indoor bonsai) are very cheap. The prices are around half to a third of a similarly sized handmade bonsai pots.
- Unfortunately, only smaller pots (usually up to 30cm, rarely above) can be produced with the casting clay technique. Larger pots are usually made by hand (with the help of plaster molds or free construction).
- The pots produced with the casting clay technique are often of poor quality compared to the other manufacturing processes. However, this is not due to the slip cast clay technique, but rather to the effort required to eliminate minor errors during manufacture (before firing). With bonsai pots for the mass market, costs are often saved here. I.e. if you want a high-quality pot, you should not buy a slip cast pot online. Individually selected in a bonsai center, you can also find good individual pots among the cheap slip cast pots.
- Although they are primarily produced for indoor bonsai, many slip cast bonsai pots are surprisingly frost-resistant. Often the value for frost resistance is around 97-98%, i.e. of 100 pots, approx. 97-98 bonsai pots survive the 1st winter. Later on, a pots rarely breaks in winter.