Discoloration and Glaze Deterioration
In dealing with antique porcelain, knowing discoloration and glaze deterioration is crucial. Knowledge of age signs can greatly help since the production of fakes has become rampant these days.
The discovery of porcelain during the Tang dynasty introduced society to materials and methods in creating tools and art.
Porcelain was always a valuable material in the past and until today. These were used in creating fixtures, ornaments, dinnerware, and even dental works.
Today, porcelain continues to serve its aesthetic purpose since these are painted and decorated pieces with patterns and images. Two notable features of porcelain that have been refined through the years are its colors and glaze.
Hence, antique items are valued for the history and prestige they represent. Porcelain, in particular, brings a different kind of prestige.
Though it is usually white, porcelain can be artificially colored. Some pieces are also adorned with paintings and patterns.
Despite porcelain’s ability to resist corrosion, time and environmental conditions make it susceptible to damage. Whether it is due to aging or handling, it is part of a piece’s true value.
Discoloration
Porcelain, much like any antique, is subject to wear and tear. This includes discoloration or stains. Discoloration and stains are caused by moisture seeping through glaze cracks or cracks in the porcelain itself.
Moisture, combined with any organic matter in the cracks, causes bacteria to grow, leading to black or brown stains. Discoloration and stains can vary, and porous materials are more susceptible to it. In this case, glazing is highly important since it almost makes a piece of porcelain perfect.
True or hard-paste porcelain is less porous than other types of earthenware. That is why many of them still survived over hundreds of years. Porcelain may be less absorbent, but time and the conditions of its environment can wear away material from a piece.
When porcelain is buried or left underwater for a long time, the glaze begins to thin. Sometimes, porcelain found on shipwrecks have no glaze left on them.
Glaze Deterioration
Porcelain and earthenware are porous in nature. Some of the first vessels made out of porcelain were for storing liquids.
But because porcelain was porous, the liquid stored in it would leak. So to stop the leakage, they developed glazing.
Glazing is a method used to seal porous materials. It uses finely powdered glass, which is coated on a vessel then fired in a kiln.
The glaze on porcelain can have a practical and aesthetic purpose. It is an added layer of protection against the environment.
However, glaze defects and deterioration are usually caused by problems during manufacturing. Though it can also be caused by aging and the environment, most of them appeared during the creation process. Here are different types of glaze deteriorations.
Defects Caused by Manufacturing
A glaze that is too thin will make a piece look rough. A glaze that is too hard won’t cover the entire surface and leave open areas. For glaze that is too soft, it’ll flow too much and create drop-like features on some parts of a piece.
Like many pieces in the Ming dynasty, about 70% of porcelain of that time had glaze contractions. These spots were either failed to be glazed or put into the kiln with oil or dirt.
Crazing
Crazing is characterized by web-like fractures usually caused by stress on the porcelain piece. It also happens when the glaze expands more than the body of the piece. Then, it is generally followed by discoloration due to moisture seeping into the cracks.
Peeling or Shivering
Peeling is the opposite of crazing. It is when the glaze of a piece shrinks, and the glaze detaches from it. Sometimes, this is due to a miscalculation of time and heat to finish the process.
Photo Source: Wiki Commons
Photo Source: Wiki Commons
Crawling
Crawling on a glaze is characterized by clumps and forms during the firing of a piece. There are usually patches where the body of a piece shows.
The glaze separates and leaves parts of the body or clay bare and exposed. This defect usually happens during the drying and cracking process.
Pinholes
Pinholes are small holes on a glaze caused by gasses not escaping fast enough during the firing of a piece. These don’t only diminish the look of a piece but can cause damage. It’s because pinholes usually reach down the piece’s body.
Blistering
Blistering are defects that occur during the firing process of a piece. Blistering is caused by gases that make the glaze bubble and harden as a piece cools. Blisters and bubbles usually break, leaving craters on the porcelain.
Unevenness
At times, insufficient coating causes unevenness. Sometimes a glaze that is too hard or soft doesn’t coat the surface of a piece evenly.
Running
Running is caused by a glaze that is too soft or when too much glaze is applied. This can cause drop-like features to appear. Thus, it looks like a running liquid.
Discoloration and glaze deterioration are present in many surviving pieces. Looking back to their techniques hundreds of years ago explains why these imperfections are present. Regardless, these pieces of porcelain are proof of art and culture that existed before us.