technical reference

Bright golds are solutions of precious metal containing organic compounds, combined with organometallic flux compounds and resin solutions.

In bright golds, the source of gold is an organogold (gold-thiol) dissolved in carefully selected solvents. Gold used to manufacture bright golds is derived from alpha pinene, copaiba balsam, and pinanyl mercaptan. Minor metals are also incorporated to control grain growth, control film formation, contribute to the perceived color, and facilitate adhesion to the substrate.

All precious metal products are mostly chemically inert and are not likely to react with ceramic colors or glazes. Adhesion occurs as a result of the substrate fusing during firing and the formation of strong physical and chemical bonds to the surface of the glaze. Adhesion of gold depends on softening of the underlying glaze and the presence of metallic fluxes (Rhodium and Bismuth resinates). The fired gold film is usually not attacked by acid and dishwashing detergents, but these chemicals do have a deleterious effect on fluxes and in time lead to gold removal.

Matte golds are also known as self-burnishing golds. Matte golds are solutions, consisting of solid precious metals, precious metal compounds, organometallic flux compounds, and resin solutions. After firing, the metal film has a matte appearance that does not require burnishing to raise a lustre; however the appearance is not as attractive or rich looking as a burnish gold.

Burnish golds are solutions, consisting of solid precious metals, precious metal compounds, and solid flux in resin solutions.

In burnish golds there are additional sources of gold used to facilitate the rich look and thickness required. Gold/Mercury amalgam powders were traditionally used to aid the addition of gold powder to the burnish phase. Today the powder added is usually pure gold with an assay approaching 100%. These sources include gold powder precipitates and insoluble resinates or gold sulphide.

After firing, a dull, non-glossy surface remains. Burnishing the surface with a fiberglass brush, sand or other means will make the layer smooth and give a silk matte finish. Burnish preparations include:

When we talk about precious metal content, we usually refer to gold, which is always the main component. However there may also be Silver, Rhodium, Platinum, or Palladium included in the formula.

Bright Golds have a gold content ranging between 6 and 24%. Preparations containing less than 6% are considered to be lustres. Preparations over 15% are rarely used. A good bright gold for decal application contains between 9 and 12% gold.

Bright Platinum and Bright Palladium preparations have a precious metal content of 3% to 12%. Although they will be white after firing, they contain more gold than platinum or palladium.

Matte Golds range in gold content from 10-18%, which is about half the gold content of a traditional burnish gold, therefore they are much less expensive than burnish gold.

Burnish Golds range from 10% to 40% precious metal. Traditional burnish golds for decal application range from 18% to 30%. High temperature, fast firing inglaze preparations require a higher gold content from 26% to 40%.

The approximate thickness of a fired bright gold preparation is 0.05 - 0.1 micron. Although the layer is extremely thin, it is very durable. For burnish gold, the layer is much thicker, approximately 0.3 - 1 micron, therefore increasing the cost, the durability, and the value of the decorated object. Matte golds fire to a thickness of approximately 0.1 - 0.3 micron. Because it is not necessary to burnish these golds, the durability is not as good as a traditional burnished gold. By burnishing, the gold is compressed and smoothed out, resulting in a very resistant layer of gold. Due to the structure of a non-burnishing gold, it is easier for detergent to attack the surface, and the gold can eventually chip off the substrate.

Precious metal selection will depend on the ware being decorated. Flux systems used in our precious metals are always designated for a specific application. For example, do not use the same burnish gold decal for leaded and unleaded bone china glazes.

Procedures: There are two separate phases in the firing of precious metals: Evaporation and firing of the gold.

In the first phase, the organic components of the preparations are volatilized and burned off. It is of utmost importance that, during this phase of firing, sufficient oxygen is present in the kiln atmosphere. By the end of this phase, at approximately 790°F (420°C), the precious metal layer (or metal oxide layer) starts to develop.

In the second phase, or firing phase, the flux additions and the metal oxides of the precious metal layers melt and interact with the silicate components of the glaze, glass, or enamel, causing mechanical adhesion. This occurs at a temperature range between 970°F to 1530°F (520°C to 830°C).

Temperature: For both firing phases, a specific time/temperature curve is required. If the kiln is heated too quickly, the quality of the finished coating can be adversely affected; for instance, a bright gold may turn dull or matte. Moreover, the required time of interaction for developing the adhesive properties will be reduced.

The fumes of precious metal preparations may cause problems in the early stage of the firing process. Because precious metal preparations are very sensitive to fumes, defects may be caused by a massive presence of such vapors. Bright gold, particularly bright lemon gold and bright platinum, may fire dull and hazy or with matte spots. A lack of oxygen prevents the precious metal from fusing with the glaze, and it can be wiped off easily after firing.

In exceptional cases, insufficient ventilation may lead to the decomposition of a decoration, particularly if the ware is loaded too closely to adequately ventilate, as for instance in a stack of plates. This may cause the contours and shapes of borders or lines to blur or even to bleed during firing. Too large a load of the kiln or racks should, therefore, be avoided, particularly in the case of large surface decorations.

Fumes should be able to escape easily. If there are not enough air vents in the kiln, it is advisable to keep the door slightly open in the initial stage of firing, up to approximately 790°F (420°C). Lehrs are therefore better than periodic kilns in this respect.

The firing of precious metal preparations requires a time/temperature curve sufficient to fuse the precious metal to the ware. When firing glass, the temperature has to be strictly monitored to avoid ware deformation. In this case, it may prove necessary to maintain the highest permissible temperature for about 10 to 30 minutes to develop sufficient adhesion.

The firing temperature required for precious metal preparations will vary according to the nature (size and composition) of the base, and depends first and foremost upon the softening point of the glaze or glass.

It is well-known that the value of gold jewelry is not only determined by its weight or thickness of plating, but also by the karat value. Occasionally, our customers ask about the karat value of a fired gold decoration. Such inquiries are often prompted by customs regulations of various countries, or to determine the legality of an advertising or sales promotion for gold-decorated glass or porcelain objects.

Except for a small amount of flux, the fired bright gold deposit contains only gold, and can be designated as 22 to near 24 karat (24 karat corresponds to 100% gold).

A bright lemon gold contains silver and less gold; therefore, the gold decoration is only around 17 karat. The karat value of burnish gold ranges between 17 to near 24 karat. The traditional yellow gold preparations have a value of 20 karat. The content of the reddish gold is somewhat higher and yields about 22 to near 24 karat, and greenish gold will be around 18 karat.

The karat values of fired gold decorations should not be considered the sole criteria of quality. The thickness of the gold decoration, as well as the gold content, should be considered. It would be a mistake to consider a plate decorated with a bright gold of nearly 24 karat more valuable than an equally decorated one with an 18 karat burnish gold. It is just the opposite; the bright gold decoration will normally be thinner, less expensive, and less durable than a well-applied burnish gold decoration.

Defect	Possible Causes
Fish Eyes	Grease or dirt on the ware. Bubbling of the glaze Dust in the decal film
Dull Film	Overfired substrate Fumes, lack of ventilation Gold layer is too thick (especially with bright golds)
Cracks	Peak temperature is too high Soak is too long Firing cycle is too short (quick heat-up and down cycle)
Blistering	Gold layer is too thick Gold / color interaction