Glass Application- Black Glass
Copper oxide or cupric oxide is a black glass pigment used in soda, lime, and silica glass colouration. It’s milled to an ultra-fine powder to allow for thorough dispersal during mixing, which eliminates speckling.
- Fine milling provides smooth and even colour
- Can be used to produce black and emerald glass on the same line
- Black colour is exposed during striking
- Contains 95%-99% CuO
- Dry bulk density: 2.6 untapped
- 98% below 75 micron/200 mesh
This container glass pigment creates heat strikable glass, which means the black colouration comes out after the annealing process when the bottles are heated to a higher temperature (600°C-650°C) to expose the latent colour. This is known as “striking”. Black glass can be made on the same production line as emerald glass by striking only the portion that requires black pigmentation.
How to Make Black Glass
Cupric oxide is the type of copper oxide that comprises up to 99% of African Pegmatite’s black container glass oxide. Its chemical composition is CuO. This container glass colorant can be used to create only black or to create black and emerald green on the same line.
Container glass is generally made of soda-lime glass. Some container glass colorants have to be developed by the application of heat, known as “striking”. Heat strikable glass contains latent colourants like African Pegmatite’s black copper container glass pigment. This glass is often called “black-strikable”. The composition may include trace amounts of additives, residual materials from cullet, and impurities typically found in glass container production.
A feeder measures out molten glass and delivers each portion to the forming machines. Each portion then undergoes press-and-blow or blow-and-blow processes by individual section machines or another suitable equipment arrangement. The containers are then annealed by appropriate equipment such as an annealing lehr at a temperature between 550° C and 600° C. The most preferable annealing is done at 550° C for an hour.
The containers may then be cooled and exit the lehr bearing the same colouration. To strike the black colouration, the temperature must then be raised to between 600° C and 680° C for up to 90 minutes. This step can take place in a secondary lehr in which the temperature is then gradually reduced to avoid damage to the new containers.
This step could also be done in the primary lehr where the initial annealing takes place, before the initial cooling. There may also be a secondary furnace adjacent to the primary where a portion of the containers undergoes the striking process.
When produced using this method, the thickness of the container may be greater than .04 inches or greater than 1 millimetre. As such, the container will appear black in indirect sunlight when viewed by the human eye at arm’s length. This means light should be transmitted at less than 10% at wavelengths between 390 nanometres and 675 nanometres. It should also transit infrared light between 750 nanometres and 1100 nanometres and can be inspected using infrared equipment.
Inspecting Using Infrared Light Devices
Until recently, it was thought that this glass, once turned black through the striking process, could not be inspected because of its low light transmission. However, this glass can be inspected using infrared inspection equipment. Infrared light is shone through the container and is received by a light sensor.
The equipment used to detect variations should include the infrared light source, the infrared light sensor, and a processor. The infrared light source may be a lamp or laser. The sensor or receiver may be a laser optic sensor that responds to wavelengths between 750 and 1100 nanometres.
The sensor will provide electrical signals to the processor, which analyses the signals to determine whether the container is acceptable. Inspection can be performed before and/or after striking to check for variations, anomalies, and defects in the walls, bottoms, heels, shoulders, and necks. Inspection may also reveal inhomogeneities that might affect the visual properties of the glass. Some variations will be acceptable while others are not.
Skilled manufacturers may be able to modify this process or the equipment used therein to achieve the same result. This process is summarised based on exemplary embodiments using container glass oxide.
If you have any questions regarding the materials that would best serve your manufacturing and other needs, please contact African Pegmatite and a specialist will be able to point you in the right direction.
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