Used in Europe and the USA, today the growth markets for bronzing are in Asia, China, India and Indonesia, where it is also used for security purposes and as an anti-counterfeiting measure.
The label or sheet of labels is printed (usually by letterpress or litho) with a special adhesive ink known as ‘prep’.
This bronze ‘prep’ (preparation) is a varnish which contains a colored pigment. Whilst the image is still wet a metallic bronze powder is applied to the surface. The ‘prep’ dries by the oxidation process thus sealing in the bronze powder.
The excess powder which has not adhered to the image is then vacuumed off the substrate and re-cycled back into the bronze powder reservoir. A final dust removal is then carried out to ensure that no Image printed using bronze prep residual powder is left on the substrate surface.
The sheet or web is then burnished using rotary burnishing bands of soft material which polishes the image giving the appearance of gold.
A further pass through the press may then be required to apply a varnish coating to the bronzed area to ensure the correct scuff resistance is met to meet agreed quality standards.
The bronzing process is illustrated in Figure 5.1 below.
Figure 5.1 - The bronzing process
In the early days of bronzing the process was carried out by hand and was generally for short run small sheet work printed on hand-fed letterpress platen presses.
A wad of cotton wool was used for dusting the bronzing onto the printed image. The powder was lightly dusted on to ensure there was no smearing of the printed image and the surplus powder was the tapped of the sheet which was left to dry, allowing the bronze powder to be fully secured to the printed image.
When the bronzing ‘prep’ is dry, the loose bronze was fully removed and the bronzed image is then burnished to give the unique gold finish. Figure 5.2 and 5.3 show examples of bronzing to produce a metallic effect on spirit labels.
Figure 5.2 - Example of early bronze label
Figure 5.3 - Example of bronzing
THE MODERN BRONZING PROCESS
The modern automated bronzing machine is a stand-alone unit which houses the bronze powder, the application system, the powder extraction and filtering and the burnishing belts.
The unit can be easily moved to operate in tandem with the selected choice of printing process and the type of press to be used for printing the ‘prep’.
The flatbed type of bronzing machine operates with the sheet in the flat position during the bronzing process.
Figure 5.4 - Modern bronzing units used for sheet-fed labels.
The sheets pass over the rubber blanket through the bronzer and the flow of bronze powder onto the tacky image is controlled through a duct similar to a conventional printing press ink duct.
A series of rotating dusting bands pass over the dusted sheet to remove the surplus bronze and burnish the image area. These dusting bands make contact with brushes which are positioned at each end section allowing the dusting bands to be brushed clean as they rotate.
A final dusting and burnishing is carried out prior to the sheets exiting the machine. This is done by rotating rollers covered with a soft material.
These machines feature a powerful internal vacuum system which draws the surplus bronze from the dusting section of the machine and the powder is filtered and collected to be used again (Figure 5.3).
Bronze powder is made from brass, zinc alloy and copper platelets. Molten metal is processed to produce tiny platelets and the platelets are graded for size, greased and polished.
The use of different metals to produce the platelets gives differing tones of bronze powder, plus copper based powders and silver aluminium. Other colors can also be produced by dyeing the platelets with colored dyes.
It is important that the bronze powder is fully secured to the printed image. Absorbent papers can be a problem as the prep can be absorbed into the substrate leaving insufficient prep on the surface of the substrate to hold and secure the bronze powder.
If the powder has not fully adhered to the prep the bronze will rub off and the image will be smeared. This damage to the printed image will occur when the sheet/web is burnished. The problem of loose bronze is also a serious problem when the label is used for food packaging. Any loose bronze powder would mean that the job would be rejected.
To help overcome the problem of loose bronze it is quite common for the bronzed image to be over-varnished. This seals the bronze powder and eliminates any loose bronze particles.
Although the bronzing process gives a ‘unique’ metallic finish it has always been considered to be a ‘messy’ process. As the bronzing powder is made up of very fine particles, airborne contamination can occur, which can lead to powder being distributed around the area of the bronzing operation and is a potential health risk. This situation has led to the development of extraction systems which are now used on modern bronzing equipment, which gives very efficient dust extraction and control.
The efficiency of the modern bronze extraction system has been confirmed by studies which have favorably compared the amount of waste generated during a bronzing operation to that from the hot foil stamping process. Worthy of note is the fact that waste bronze powder, which consists of 85% copper, has a resale value and can be sold to the recycling industry.
The issue of loose powder has prompted the label industry to seek out alternatives to the bronzing process. In recent years, development work has been carried out which allows the replacement of the traditional bronzing process with a conventional printing processes.
This development involves immersing a standard bronze powder into a conventional ‘solvented’ clear varnish ink system and then printing the bronze ink using a conventional screen process. This method eliminates the requirement for a secondary varnish pass to seal the bronzed image whilst overcoming some of the environmental/health and safety issues surrounding the use of loose bronze powder.
Developments using bronze effect foil applied using hot or cold foiling have produced acceptable results, but these systems do not fully achieve the unique effect created by the traditional bronzing process.
A technical solution developed by Matheoschat allows a web-fed bronzer to be integrated into a self-adhesive label printing press. This unit is capable of delivering specially developed gold and silver prep onto a variety of substrates.
New powders are also available which give new shades of gold, and silver plus pearlescent and other special effects.
Within the sheet-fed market there is a demand for small bronzing machines that cater for the small format, short run markets.
Mobile lightweight bronzing units have been developed that can be connected and disconnected to a printing press quickly. After the bronzing work is completed the unit can then be removed from the press.