Introduction
Glass is a versatile material that has been used for thousands of years. It can be formed into many shapes and sizes and has a wide range of applications. One of the interesting properties of glass is that its colour can be modified, allowing glassmakers to produce glass in different colours. There are several ways to change the colour of glass.
Add Colourants
The most common way to colour glass is by adding colourants, also known as colouring oxides. These are compounds of certain metals that provide distinct colours when added to molten glass. Some examples of common colourants used in glassmaking include:
- Iron oxide – for green, brown, or grey glass
- Copper oxide – for turquoise or blue glass
- Gold chloride – for ruby red glass
- Manganese dioxide – for purple glass
- Cobalt oxide – for deep blue glass
The colourant is added to the raw materials during the glass melting process. As the glass melt is heated to high temperatures around 1000-1200°C, the colourant dissolves and integrates into the glass mixture. The amount of colourant added controls the depth and shade of the colour produced. Glassmakers can blend different metal oxides in various proportions to achieve the desired colour effect.
Change Glass Thickness
Another way to modify the colour of glass is by changing its thickness. Thicker sections of glass appear darker in colour than thinner sections. This is because as light travels through glass, some of it gets absorbed by the material. The farther the light has to pass through, the more absorption occurs, causing thicker glass to take on a deeper colour.
For example, a piece of clear glass can show a pale green tint when it is made into a thicker section. On the other hand, thin sections of traditionally coloured glass can appear much lighter than normal due to less light getting absorbed as it travels through the minimal material.
Using variable thickness and layering clear and coloured glass is an artistic technique exploited by glassmakers to achieve desired optical effects. Stained glass windows are a prime example of using glass thickness to control colour depth.
Heat Treatment
Heating glass to high temperatures can also permanently alter its colour. When clear soda-lime glass is heated to around 600°C for extended time periods, sodium ions in the glass composition move around, causing the glass to darken slightly. The longer the exposure time, the darker the colour becomes, creating amber, brown, or even black glass.
Heating also affects the structure of coloured glass, leading to colour changes. For instance, copper ruby glass can turn into a deep opaque red when subjected to heat treatment. The heat causes the copper ions to cluster together, increasing light absorption and creating a bold red appearance. This technique is used by glass artists to create vibrant colours in glass artworks.
Irradiation
Exposing glass to radiation can introduce colour centres that absorb light, causing colour changes. Gamma rays, X-rays, and electron beams are commonly used to irradiate and alter the colour of glass for research and artistic purposes.
When clear soda-lime glass is irradiated with high energy rays, electron transitions within the material result in the generation of brown and pink tints based on the radiation dosage. This method allows controlled colour modification of glass.
Irradiating coloured glass also leads to distinct changes in shade and depth of colour. For example, cobalt blue glass turns to olive-green when subjected to gamma rays. By carefully controlling radiation levels, glassmakers can achieve precise colour alterations.
Surface Coatings
Coating the surface of glass with coloured synthetic resins can also change its apparent colour. This technique involves dipping clear glass into a solution containing dye dissolved in a resin binder. Once the coating dries and hardens, it forms a translucent coloured layer on top of the glass.
Common resin coatings used to colour glass include perylene dyes, metal oxide pigments, and coloured silver halide salts. Multiple coats can be applied to increase the depth of colour. The coatings allow for vibrant colours like bright red, blue, and yellow to be achieved without affecting the glass composition.
Effects of Colour Change on Glass Properties
Modifying the colour of glass through various techniques can impact some of its properties:
- Adding colourants alters the chemical structure of glass, which affects workability and melting temperatures.
- Heating may decrease strength and increase brittleness in glass.
- Irradiating glass can induce defects and weaken the glass structure.
- Coatings change the surface properties like glossiness and texture.
However, in most cases, the bulk physical and mechanical properties of the underlying glass remain unchanged. With careful control of processing parameters, high-quality coloured glass can be produced.
Types of Coloured Glass
Several distinct types and styles of coloured glass exist due to the various colouring techniques:
| Type | Description |
|---|---|
| Stained Glass | Coloured and painted glass used for decorative windows, incorporating coloured glass, paint, and staining. |
| Dichroic Glass | Glass with a thin transparent coating that produces interference colours by reflecting specific light wavelengths. |
| Glass Enamels | Finely ground coloured glass powders that are melted onto glass objects to produce a coloured glossy surface. |
| Flashed Glass | A layer of coloured glass melted on top of another glass colour, often clear. |
Applications of Coloured Glass
The ability to modify glass colour has led to numerous applications leveraging the aesthetic qualities of coloured glass:
- Stained glass windows and decorative glass artworks
- Coloured glassware – plates, bowls, vases
- Ornamental glass objects
- Architectural coloured glass panels and walls
- Automotive glass
- Specialty coloured lenses and filters
- Laboratory glassware
Vibrant colours allow glass objects to possess an artistic quality. The variety of shades and hues achievable by colouring glass continues to provide opportunities for innovative applications across many fields.
Conclusion
Changing the colour of glass is made possible through several techniques that alter its optical properties and composition both internally and on the surface. While adding colourants is the most dominant method, glass colour can also be controlled by changing thickness, heating, irradiation, and coatings. Varying these parameters allows glassmakers to produce stained glass, dichroic glass, enamels, and other distinct coloured glass types and articles. The ability to tailor glass colour has led it to be an integral material for art, architecture, crafts, and engineering applications throughout history. Continued glass research and artistic approaches ensure coloured glass remains an exciting material into the future.
