My system is not decent

Chapter 1159
The percentage of volume reduction of porcelain after firing is called firing volume shrinkage.

Of course, in porcelain, the main purpose is to calculate the firing shrinkage ratio so that we can accurately grasp the size of the porcelain after firing.

In ancient times, this could only be roughly estimated by relying on accumulated experience.

But in modern times, there is indeed a calculation formula.

The traditional method is to shape the finished powder under a specified pressure isostatically, then burn it in a kiln, and then measure the shrinkage.

Then according to the shrinkage rate and the specified pressure, adjust the size of the green body appropriately to organize the production.

However, due to differences in equipment, fluctuations in raw materials and milling processes, the shrinkage rate is constantly changing, and fluctuations in the size of the porcelain body should be inevitable.

Generally, when the green body is formed, it is more scientific to control the shrinkage rate by adjusting the pressure to control the compaction density, which is more scientific than simply controlling the pressure to ensure shrinkage.

In addition, during the powder manufacturing process, the loss on ignition of various raw materials can be controlled to ensure the stability of the loss on ignition.

As long as the loss on ignition is stable, the shrinkage can be calculated from the compacted density.

These are a bit complicated, anyway, you have to know that the shrinkage ratio of porcelain firing is still very important.

Where is that important?Isn't it just boiled dry, with less water and reduced volume?

That being said, regardless of the shrinkage ratio, the possibility of porcelain burning is still very high.

Leaving aside the one-to-one imitation of ancient porcelain, it is necessary to calculate the shrinkage ratio.

There is also the need to consider the shrinkage ratio when repairing porcelain and re-firing it in the kiln. Chen Wenzhe had specially studied this when he repaired Longquan kiln porcelain before.

This is also an important reason why he didn't pay too much attention to the firing shrinkage ratio before, because he can control the shrinkage ratio well.

The first thing to do to make porcelain is wet mud molding. Whether it is drawn, printed, or grouted, it will shrink during the drying process.

The large shrinkage rate may cause cracking at the bottom of the billet. Therefore, someone needs to use thin iron wires to continuously divide the supporting base plate and the billet body within a fixed period of time to ensure the safe shrinkage of the billet body.

After the porcelain is fired, the shrinkage rate is different. The reason is that the firing temperature is different, the raw material of the green body is different, and the molding method is different. The comprehensive factors are caused.

Jingzhen’s traditional standard is “one foot and eight inches left”, that is, if you want to make a piece of porcelain with a diameter of eight inches, you must make it a diameter of one foot when molding, which is the shrinkage rate after firing.

Jing Zhen's calculation method lasted for a long time.

Therefore, when Jingzhen fired porcelain, it was necessary to add a cushion cake made of the same mud as the body under the bottom of the vessel to increase the contact surface between the vessel and the bottom of the sagger.

This will reduce the friction with the bottom plate of the sagger, so as to ensure that the foot of the green body can slide when it is fired and shrinks, so as to avoid the bottom plate of the utensil from being cracked.

Using this calculation method, the error can be controlled below two percent.

The shrinkage ratio of porcelain fired in other places is generally different from that of Jingzhen porcelain.

The reason for the gap is mainly due to the different types of porcelain. Strictly speaking, Jingzhen high-end porcelain should be called "Chinese fine porcelain".

Its firing temperature is above 1280 degrees, and the firing temperature of some porcelains generally reaches 1240 degrees or lower, so it cannot be called "fine porcelain".

Because the density and strength of the carcass is lower than that of Jingzhen porcelain, its shrinkage rate must be smaller than that of Jingzhen porcelain.

Therefore, the data of one foot and eight inches cannot be used as data on the shrinkage rate of porcelain in other regions.

Similarly, the shrinkage rate of low-grade porcelain produced in Jingzhen is smaller than that of high-grade porcelain.

This is where the problem arises. The shrinkage rate of high-grade porcelain is so high that it will indeed cause deformation of the picture decoration, but it will not cause the glaze to form an orange peel glaze.

When the glaze is in a liquid state at high temperature, it will naturally flow smoothly.

The orange peel glaze will only appear if the temperature has not reached the melting point of the glaze, but it has nothing to do with the shrinkage.

As the body shrinks, the picture will inevitably shrink.

If it is on a plane, the lines of the decoration will be more refined and the composition will be more compact.

Is the shrinkage rate in porcelain making, all benefits?
Obviously it is impossible, if you compose on a corner or slope, the pattern will be deformed.

Only by understanding these changes and mastering the proportion of the changes, especially the painters mastering this shrinkage phenomenon, will they try to find relatively flat parts to reduce deformation when drawing figures, animals and other decorative patterns with high modeling requirements.

Therefore, in the corners, slopes and other parts, it is necessary to handle with care.

For example, draw some small corner patterns, so as to avoid the problem of pattern deformation.

In addition, the space for composition must also be reserved in advance.

Generally, before mass production, samples must be made, that is, trial firing.

Chen Wenzhe's trial firing before was all to see the effect of the glaze color.

In fact, sometimes he will ignore many details, because he has mastered these details and is sure that there will be no problems in these details, so he doesn't pay much attention to them.

When he was making a piece of porcelain, every process was obtained from inheritance, and it was a technique honed by some of the top craftsmen.

Therefore, even if he doesn't pay attention to many small details, he can do well.

This is a good foundation, and he will not make any low-level mistakes at all.

Of course, the problems that don't happen to Chen Wenzhe don't mean they won't happen to other people.

To be honest, some small details in porcelain making are still very important.

For example, try firing to check the shrinkage ratio. The very important reason is to determine the effect, deformation and space of the decoration after firing.

Once the decoration is determined, no one dares to change a type of utensil for decades.

Chen Wenzhe has mastered the shrinkage ratio, so he will be very sure of the final firing effect of his own works.

Of course, porcelain not only shrinks but also expands during firing.

Expansion occurs because the weight of the upper part of the utensil is pressed, causing the utensil to deform, expand or sag.

There are still quite a few porcelains that are relatively prone to this expansion phenomenon, such as Tianqiu vases and appreciating vases.

Because of these types of porcelain bottles, there is a protrusion on the top that presses the bottle body, causing the belly of the bottle to expand outward.

However, due to the large shrinkage rate, the finished porcelain is generally not larger than the diameter of the green body, but it will expand relative to the height.

These are the guarantees for the success rate of firing. If you don't understand these, it is very normal for the final firing to be deformed and collapsed.

This is the limitation of many folk kiln works. In addition, in fact, there is no difference in shrinkage between official kiln wares and folk kiln wares.

However, due to the fact that official kiln wares are generally made of better materials, the firing temperature is high, and the density of the porcelain is high, so it is naturally heavier.

This phenomenon is what we often refer to as "heavy hands". The reason why it is heavier is naturally because the shrinkage rate is greater than that of ordinary folk kiln wares.

However, this difference is not so large that it is difficult to detect by visual inspection alone.

(End of this chapter)