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 based on accumulated experience.
And in modern times,
There is indeed a calculation formula.
The traditional method is to isostatically press the prepared powder under a specified pressure, then bake it in a kiln, and then measure the shrinkage rate.
Then the size of the green body is appropriately adjusted according to the shrinkage rate and the specified pressure.
to organize production.
However, due to differences in equipment, fluctuations in raw materials and milling processes, the shrinkage rate is constantly changing.
Fluctuations in the size of the porcelain body are unavoidable.
Generally, during green molding, it is more scientific to control the shrinkage rate by adjusting the pressure to control the compaction density, rather 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 rate can be calculated through the compaction density.
These are a bit complicated, but you have to know that the shrinkage ratio of porcelain during firing is still very important.
So what's important? Isn't it just that it is dried out, the water content is reduced, and the volume is reduced?
Having said that, regardless of the shrinkage ratio, the possibility of burning the porcelain is still very high.
Let’s not talk about one-to-one imitation of ancient porcelain. We need to calculate the shrinkage ratio.
When porcelain is repaired and fired in the kiln again, the shrinkage ratio also needs to be considered.
Before this, when Chen Wenzhe was repairing Longquan kiln porcelain,
I have done special research on it.
This is also an important reason why he did not pay too much attention to the firing shrinkage ratio before, because he can control the shrinkage ratio well.
To make porcelain, wet mud must first be formed. Whether it is drawn, printed, or grouted, shrinkage will occur during the drying process.
A large shrinkage rate may cause the bottom of the blank to crack, so someone needs to continuously separate the supporting plate and the blank with thin wires within a fixed period of time to ensure the safe shrinkage of the blank.
After porcelain is fired, the shrinkage rates are different. The reason is due to different firing temperatures, different green body raw materials, different molding methods and other comprehensive factors.
The traditional standard in Jingzhen is "one foot leaves eight inches", that is, if you want to make a piece of porcelain with a diameter of eight inches, you must make it one foot in diameter when molding. This is the shrinkage rate after firing.
Jingzhen’s calculation method lasted for a long time.
Therefore, when Jingzhen fired porcelain, he would put a pad made of the same mud as the base of the object under the base of the object to increase the contact surface between the object and the bottom of the sagger.
This will reduce the friction with the bottom plate of the sagger.
This is to ensure that the feet of the green body can slide when it shrinks during firing to avoid cracking of the bottom plate of the utensil.
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 main reason for the gap is the different varieties of porcelain. Strictly speaking, Jingzhen's high-end porcelain should be called "Chinese fine porcelain".
The firing temperature is above 1280 degrees. The firing temperature of some porcelain is generally 1240 degrees or lower, so it cannot be called "fine porcelain".
Because the density and strength of the matrix are lower than those of Jingzhen porcelain, its shrinkage rate must be smaller than that of Jingzhen porcelain.
Therefore, the data of eight inches per foot 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.
A problem arises here. The shrinkage rate of high-end porcelain is so high that it will indeed cause the deformation of the picture pattern, but it will not cause orange peel glaze to form on the glaze.
When the glaze is in a liquid state at high temperatures, it will naturally flow smoothly.
Orange peel glaze will only appear when the temperature does not reach the melting point of the glaze, but it has nothing to do with the shrinkage rate.
As the vessel shrinks, the picture will inevitably shrink.
If it is on a flat surface, the lines of the decoration will be more refined and the composition will be more compact.
Does that mean that shrinkage is a benefit in porcelain making?
Obviously it's impossible. If you compose the picture at a corner or on an inclined surface, the pattern will be deformed.
Only by understanding these changes and mastering the proportions of changes, especially if painters have mastered this shrinkage phenomenon, can they try to find relatively flat parts to reduce deformation when drawing figures, animals and other highly demanding patterns.
Therefore, you must be careful when handling corners, slopes, etc.
For example, draw some small corner patterns 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 burning.
Chen Wenzhe's previous test firings were all to see the glaze effect.
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 does not pay much attention to them.
When he makes a piece of porcelain, every process is inherited and is a technique honed by some of the best craftsmen.
Therefore, even if he doesn't pay attention to many small details, he can still do a good job.
This means that if he plays the basics well, he won't make any low-level mistakes at all.
Of course, problems that do not occur in Chen Wenzhe do not mean that they will not occur in other people.
To be honest, some small details in porcelain making are still very important.
For example, test firing to check the shrinkage ratio, a very important reason is to determine the effect deformation and space after the pattern is fired.
Once the decoration is determined, no one dares to change the shape of the vessel for decades.
Chen Wenzhe masters the shrinkage ratio here, so he will be very sure of the final fired effect of the work he made.
Of course, during the firing stage, porcelain not only shrinks but also expands.
Expansion occurs because the weight of the upper part of the utensil presses on it, causing the utensil to deform, swell, or sag.
There are still many porcelains that are prone to this kind of expansion phenomenon, such as celestial sphere bottles, appreciation bottles, etc.
Because these types of porcelain bottles have a protrusion on the top that presses the body of the bottle, causing the belly of the bottle to expand outward.
However, due to the large shrinkage rate, the porcelain will generally not be larger than the diameter of the green body, but will expand relative to the height.
These are the guarantees of the success rate of firing. If you don’t understand these, it is completely normal for it to deform and collapse after firing.
This is the limitation of many folk kiln works. In addition, in principle, there is no difference in shrinkage between official kiln wares and folk kiln wares.
However, because official kiln wares generally have better materials, higher firing temperatures, and higher porcelain density, they are 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 kilns.
However, this difference is not large and is difficult to detect by visual inspection alone.