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Technical Support

 
925 Sterling Silver
  1. Red stain
  2. Porosity
  3. Rough Surface
  4. Tarnishig
  5. Precipitation hardening

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 KW Jewelery
  1. Hard Spot
  2. Oxide Porosity
  3. Gas Porosity
  4. Impurities Inclusion

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 KR Jewlery
  1. KR Cracking

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Red Stain

After casting, there are red-stain patches found on the surface of the castings. Even though polishing, the red-stain can be removed only rarely. Even by electro-plating, the red-stain still cannot be effectively. Normally, red-stain associated with some tiny pores was caused by Copper I Oxide and Copper II oxide. Its presence of copper oxide, red to black color appears. Silver is known as inert metal that would not react with oxygen. However, the oxygen absorbed by the molten silver will react with the silver alloy to form corresponding oxides.

Solution

To minimize the red-stain, the alloy casting should be melted under protective gas such as, Argon or Nitrogen. The old scrap should be cleaned thoroughly by pickling and tumbling to remove the oxide and investment powder before re-melting. If the old scrap is found heavily red-stained, the alloy is no longer suitable for casting. The alloy should be refined immediately.

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Porosity

The pore are small, clouded and with a spherical shape. By naked eye, it is hardly to recognize the pores. It will appear as a cloudy surface accompanying with red stain. Because of the low melting point of the silver alloy, the shrinkage porosity is rarely occurred. In most case, the gas porosity is hardly to eliminated by intensive polishing. The main reason cause this problem is the scrap contamination during the melting process and the chemical reaction is as follows:

Ag2S/CuS + O2 R 2Ag/Cu + SO2 -

The reaction between the investment and the melt during the casting is another cause.

Solution

To avoid this defect, the recycled scrap should be used as less as possible to avoid contamination. Also, before to use, the scrap must be ensured that the investment powder is completely by tumbling.

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Rough Surface

For silver jewelry, heavy items are the most hot topic in this new trend. That means the surface area per volume ratio is reduced. As a result, the retained heat would thus favor the metal/investment reaction as following:

CaSO4  +  Cu  ®  CaO  + CuO  +  SO2

Solution

To avoid this pinhole, the melt temperature and the flask temperature should be reduced to certain level until the pinhole problem stopped.

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Tarnishing

The tarnishing was caused by one or more than one chemical reaction, which affected the silver alloy surface with unacceptable dark pattern. Silver is known as the most reactive inert metal, it react very easily with sulfur-containing compounds, with formation of corresponding sulfides, only 1/600000000v/v (one of six hundred million) of sulfur would cause the tarnishing. The simplified tarnishing reaction formula is as followings:
2Ag + H2S + 1/2O2 R Ag2S + H2O

Solution

1. Electroplating
Plating with Rhodium provide the good protective layer but the cost is quite high. Also, the texture of the product will be much affected.
2. Organic Layer
Covering the silver jewelry with organic layer separate any contact between sulfur in atmosphere and silver. However, the colorless organic layer will affect the texture of the jewelry.
3. Change alloy composition
Some special additives will prevent the formation of sulfur-containing compounds. It will slow down the tarnishing and change the mechanical property of the silver alloy.

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Sterling Silver Hardening

Pure silver of about 30HV is too soft for most jewelry application. That's why the addition of other base metal such as copper is done to achieve a harder material. The normal sterling silver of about 60HV is sufficient for jewelry application. It is too soft in the application of clasps and some spring items. For most serious case, the jewelry may result in undersirable deformation.

Solution

To improve the hardness of silver jewelry, heat treatment process, so called, Precipitation Hardening can be done. The procedures are as follows:

1. The alloy is heated up to 550oC in furnace with protective gas cover for 5-10 minutes and then quenched in water immediately. If the jewelry items have not subjected to any cold-working, this step can be ignored. The objective for this quenching process is to obtain the super-saturated solid solution of the copper-rich phase.

2. The casting items are then re-heated to 280-300oC in furnace for 1 hour with protective gas. The items are then cooled on iron plate. The step is to provide desired amount of energy to allow the precipitation of the copper taking place. If the temperature is too low, the precipitation cannot take place. In contract, if the temperature is too high, too energy is supplied that cause the copper-rich phase re-dissolve into the silver phase and results no hardening effect.
Normally, the hardness can be increased about 50% to about 90HV after the treatment. However, the ductility will be lowered.

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Hard Spot/Steel Sand

Hard and brownish particles termed as "hard spot" are usually found in the white gold casting. Those impurities are very difficult in polishing. In general, the alloy contains lesser zinc and the presence of silicon has a higher risk of getting such hard spot problem. The detect can be found in both Ni-white and Pd-white gold alloy.

Solution

Limiting the percentage of scrap usage (not more than 30%) and melting the alloy with protective gas to minimize the oxide appearance.

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Oxide Inclusion Porosity

Oxide inclusion porosity resembles shrinkage porosity but it is caused by the low content of zinc oxide. Zinc metal is usually added (10-20%) to reduce the melting point and increase fluidity. In contrast, the defect of Pd-white alloy is not so significant because of the limit amount of the zinc usage. In actually, too much zinc content will cause the alloy becomes brittle.

Solution

Covering the alloy with protective gas and yellow flame during melting, limiting the old scrap usage(not more than 30%), avoiding to use torch for melting.

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Gas Porosity

Gas porosity is commonly found in Ni-white gold casting, but rare in Pd-white gold casting. The gas porosity is due to the sulfur dioxide evaluated from reaction between metal sulfide and oxygen or oxides:

M'S + 2M"O/O2 ® M' + 2M" + SO2­

Beside of the re-melt scrap, new raw material may also contain significant amount of nickel sulfide due to the refine of nickel metal.

Solution

Similar to Hard Spot, limiting the percentage of scrap usage (not more than 30%) and melting the alloy with protective gas to minimize the oxide appearance can significantly improve the problem.

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Impurities Inclusion

Both Ni-white and Pd-white alloy have a relatively higher melting temperatures. That means they would solidify faster. If the sprue sets in improper position or too thin, the impurities will be trapped in the item rendering some red/black stain and porosity problem, even cracking.

Solution

To avoid the defect, the casting temperature is increased to enhance the fluidity of the melt. So that, the impurities can leave the casting item prior to solidification. Recycling scrap must be cleaned before use.

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KR Cracking

The 18K red gold is found to be most brittle and fractured. The alloy is broken without any markable deformation. The defect happened in mainly pink alloy and some gold alloys with high copper content.

Solution

  1. Those cracking can be avoided by quenching the alloys from the temperature above 400oC and the alloy retains its ductility with low hardness. During soldering, the alloy may be heated up. Once slow cooling occur, the ordering will undergo. Therefore, the alloy should be quenched to retain its ductility after soldering.

 

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