Common reasons for blackening of gold-plated jewelry and cleaning methods
April 19,2022
Gold-plated jewelry is a common decorative jewelry in our life, but in daily wear, it is often found that gold-plated jewelry will change color or even turn black after a period of time. What is the reason for this? Today, we will conduct a simple analysis on the material characteristics of gold-plated jewelry.
Gold-plated jewelry in this analysis refers to gold-plated jewelry with heterogeneous materials. As the name suggests, it is plated with a layer of gold on the outside of other metal embryos, while pure gold and pure K gold are not within the scope of this discussion. That is to say, the golden color of the outer layer is not the color of the base of the embryo itself, but it relies on technical means to attach a layer of gold to the surface, so it will definitely fade when worn for a long time, just like cotton and linen clothing - it will wrinkle, cashmere T-shirts - the same reason that they will pilling, are material characteristics, please treat them rationally. Only due to the thickness of the coating, technology and maintenance habits, etc., the color retention time is different.
Common gold-plated jewelry materials generally divided into copper gold-plated and silver gold-plated.
Copper gold plating: copper base + k gold plating;
Silver gold plating: silver base + k gold plating.
K gold plating is generally divided into 14k gold and 18k gold, which is composed of gold, silver, copper and zinc. The biggest difference between the two is that the gold content is different. The content of 14k gold is 58.5%, and the content of 18k gold is 75%, so generally 18k gold will be darker and more yellowish than 14k gold.
According to the chemical metal activity sequence table,
Chemical stability: gold>silver>copper>zinc,
The chemical properties of gold are very stable, and it does not react chemically with most substances, except that gold reacts with mercury.
The physical and chemical properties of silver are relatively stable, but silver easily reacts with sulfur to form silver sulfide, which is gray-black.
Copper is a less reactive heavy metal, but after a long time in humid air, a layer of patina (basic copper carbonate) will slowly form on the copper surface. The chemical properties of zinc are active, and in the air at room temperature, a thin and dense basic zinc carbonate film is easily formed on the surface.
We first analyze the reasons for blackening from the external coating
1. Generally, the coating of gold-plated jewelry is thin, and the coating is easily worn for a long time and exposes the base, causing the jewelry to change color;
2. Long-term wear, because the gold-plated jewelry will be polluted by a lot of dust, dirt, grease and other impurities, causing the gold-plated jewelry to turn black;
3. When the gold-plated jewelry comes into contact with other substances, the chemical reaction will cause the jewelry to turn black, such as sweat, cosmetics, etc.
From internal matrix analysis
The discoloration of gold-plated jewelry may also be caused by the diffusion of the matrix through the pores of the gold-plated layer to the gold-plated layer.
How to clean blackened gold-plated jewelry
1. When the gold-plated jewelry is only slightly changed, we can clean it with toothpaste or silver cloth;
2. If the jewelry has completely turned black or green, that is, the gold-plated layer has been damaged, it is difficult to restore it unless it is re-plated.