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Clay Plaster molds Kilns The fire Production techniques
Production of slip cast bonsai pots Production of handmade bonsai pots Yixing bonsai pots

The production of bonsai shells is carried out using different techniques (e. g. B. casting clay technique or made by hand) with very different results concerning the quality of the bonsai pots.

Despite the great differences in the results, the production of bonsai pots is very similar for all techniques: the clay and the plaster moulds are bought or produced independently, the clay is (usually) shaped with a plaster mould, the pot blank is reworked and dried, possibly glazed and then fired.

The clay

Most of the bonsai pots available in stores are made of ceramic. Such ceramic bonsai pots are made of clay. The clay used for this can have very different properties:

  • Fireclay: Is added to the clay of large bonsai pots so that they do not warp during drying and firing. The proportion of fireclay in large pots is often in the range of 20-25%.
  • Color: The clay color varies greatly. Often dark brown clay is used. But the use of gray to almost black clay is also common. Even yellow and blue clays are used from time to time.
  • Firing temperature: Each clay has a certain temperature at which it sinters. During sintering, the ceramic base materials of the clay "bake" together at a certain temperature in such a way that the ceramic absorbs little or no water afterwards. The less water the finished ceramic absorbs the more resistant the ceramic is to frost damage. Typical sintering temperatures for bonsai pots are in the range between 1100-1220°C.

The clay is usually purchased from specialized companies. In some bonsai potteries, the clay is also still made by hand. To do this, the clay is ground, sieved and, after adding some water, pressed into plates or bars.

The plaster moulds

The clay used is brought into shape with the help of plaster moulds (exception: free assembly by hand), more or less elaborate finishing (smoothing, glazing, possibly decorate or engraving) and dried.

The production of the plaster moulds is not that simple and is therefore carried out by specialised employees. Usually a "master mould" is made from plaster first. From the master mould, the "working moulds" are produced from plaster.

Each working mould can be used to produce several dozen identical bonsai pots. If new working moulds are required, these can be created at any time with the help of the master mould.

The ceramic kilns

After drying, the raw ceramics are fired in a ceramic kiln. Such clay ovens were often heated with coal in the past. Nowadays, gas or electric kilns of various sizes are more common. Usually the clay is only fired once.

In the case of glazed bonsai pots, the blank is often fired twice (1st firing up to approx. 950°C -> glaze is applied -> 2nd firing at approx. 1100-1220°C)

The firing

Regardless of the clay or kiln used, the firing of the bonsai pots is similar and is divided into the heating phase, holding time at the final firing temperature and the cooling phase.

Above all, the duration of the heating phase and the holding time at the final firing temperature are very important factors in determining whether the result is good or whether the fired bonsai pots must be discarded.

Since the time for the phases can be very different, it makes little sense to list too much data here. Just a typical example that is used for many medium-sized handmade pots in gas kilns:

  • Heating at approx. 80-100°C per hour up to a firing temperature of 1200°C. Duration approx. 12-14 hours.
  • Holding time at 1200°C approx. 30 minutes. During this time, the bonsai pot sinters and a glaze liquefies and takes on its final color.
  • Cooling down phase: Often takes 24-30 hours and depends largely on the insulation of the kiln. A well-insulated kiln cools down slowly. That's a good thing. If a ceramic cools down too quickly, it can burst or crack.

During the heating phase and the holding time, the clay changes in different steps in such a way that in the end result our bonsai pot keeps its shape, is no longer water soluble (clay can be dissolved in water) and can even withstand the deepest frosts.

  • Heating phase up to 100°C: Although the pots have been dried, they still contain a lot of water. This water evaporates up to 100°C. If it is heated up too quickly, the bonsai pot can already be torn by the steam pressure.
  • Heating phase 100-400°C: Further (crystalline bound) water escapes. Up to this point the pot is still water-soluble. I.e. if you took it out of the oven and put it in water it would completely disintegrate.
  • Heating phase 400-950°C: The clay changes chemically step by step. With many types of clay, this process is completed at approx. 950°C. Now the clay is no longer soluble in water. The fired ceramic is still very porous and can absorb a lot of water. I.e. the pot is still very brittle and not frost-proof.
  • Holding phase at approx. 1100-1220°C: Now the clay sinters and becomes more and more dense and solid. If the temperature and holding time are right (varies according to the clay), the bonsai pot becomes frost-proof because it can hardly or not at all absorb any water.

If a ceramic is fired too long or at too high a temperature for the clay used, the ceramic may melt in the furnace. If the duration is too short or the temperature is too low, the pot does not sinter completely and is then not completely frost-proof.

Burning a bonsai pot is a very complex process. Even with experienced potters, mistakes can quickly occur during firing because, for example, in the kiln the temperatures on the wall differ from the middle or because more air (oxygen) penetrates at one point and possibly changes a glaze.

How then can you distinguish a good pottery from a bad one ? Quite simply - due to the size of the pile of scrap ;-) With good potteries the pile is large, the bad potteries also sell the rejects.

Production techniques

Bonsai pots are made in different ways. They can be produced by hand (free construction or with the help of plaster moulds) or in an industrial way by pressing or pouring the clay.

The production methods for bonsai pots are all justified. The result are bonsai pots that differ significantly in quality, price and area of application.

The following manufacturing techniques are used in the production of bonsai pots:

  • Slip cast technology: The slip casting clay technology is used for the production of inexpensive bonsai pots. Its low cost makes it suitable for the mass production of bonsai pots for indoor bonsai. The pots produced with the slip cast technique are often of poor quality. The main advantage is their low price. Although they are primarily produced for indoor bonsai, many slip cast 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 bonsai pot rarely breaks in winter.

    More information at: Production of slip cast bonsai pots

  • Pressing technique: The clay pots are produced with a press. That usually goes very quickly. With smaller pots, a good worker often only needs 20 seconds. With this technique, too, bonsai pots can be produced extremely cheaply and it is very suitable for mass production. And mass production really means mass production in China. One pottery we visited produced many millions of pots and saucers a year. Another advantage of the pressing technique is: The bonsai pots are often much more accurate and dimensionally stable. Disadvantage: Only bonsai pots can be made that are wider at the top. Otherwise it is impossible to get the bonsai pot out of the mould.

  • Slip cast pressure technology: Here the liquid clay is pressed into the moulds under pressure. The resulting pots are very accurate and dimensionally stable. This technique is also suitable for making large quantities of pots. Since the technology is new, the price (due to the high investment costs at the beginning) is significantly higher than for the slip cast or press technology. However, the price is likely to fall in the next few years due to increasing competition. The quality of these pots is very good. Unfortunately, at the moment (2020) almost only smaller pots up to 30cm are produced.

  • Handmade with the help of plaster mould: This technique is particularly used for bonsai pots >30cm, as large bonsai pots are difficult to manufacture with the other techniques (casting, pressing, slip cast pressing). Handmade does not automatically mean high quality. The quality of the pots depends to a large extent on the care that is taken in the post-processing of the still unfired pot blanks. Depending on the bonsai pot size and quality requirements, a worker here often only produces 5-10 pots a day, and often even less if the pots are very good. This is also reflected in the price. Good, handmade bonsai pots cost many times more (often ten to twenty times more) than an industrial pot and are usually good frost-resistant.

    More information at: Production of handmade bonsai pots

  • Handmade without plaster mold: We have now reached the top quality class. The free construction of a bonsai pot without the aid of a plaster mold is very complex. Often a good potter creates only one pot a day, which is reflected in the very high price. But you also gets an individual pot for your bonsai, often with a dedication under the bottom of the pot. Such bonsai pots should be 100% frost-resistant.

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