Ceramic tiles come in a wide variety of colors and patterns. The process of coloring ceramic tiles is an intricate one that involves several steps during the manufacturing process. In this article, we will take a deep dive into understanding how ceramic tiles get their color and vibrancy.
The Raw Materials
The starting point for colored ceramic tiles lies in the raw materials used. Clay is the primary component of ceramic tile bodies. The natural color of clay depends on the minerals present in it and can range from white to red and brown. To produce ceramic tiles in different colors, clays with different mineral compositions are blended together.
Some commonly used clays for ceramic tiles include:
| Clay Type | Color |
|---|---|
| Ball clay | White |
| Kaolin clay | White |
| Fire clay | Red, brown |
| Stoneware clay | Grey, brown |
In addition to clay, other ceramic raw materials like feldspar, silica, and alumina are also added to the tile body formulation. These materials help control the working properties and firing behavior of the ceramic tiles.
Adding Color
The next step is introducing color to the raw ceramic tile body. There are two main approaches used:
1. Using naturally colored clays
2. Adding ceramic pigments
Naturally colored clays
As mentioned above, clays have inherent colors based on their mineral content. For some ceramic tiles, manufacturers may use a specific colored clay to achieve the desired hue. For example, a red firing clay can be used to produce reddish ceramic tiles.
However, natural clays alone provide a limited palette of colors. Most ceramic tile manufacturers use ceramic pigments to color the tile bodies.
Ceramic pigments are inorganic compounds that provide vibrant, stable, and long-lasting color. Common pigments used include:
| Pigment | Color |
|---|---|
| Cobalt oxide | Blue |
| Chromium oxide | Green |
| Iron oxide | Red, brown, yellow |
| Nickel oxide | Brown |
| Copper oxide | Green, red |
These ceramic pigments are added to the tile body in powdered form. The amount of pigment added controls the intensity and saturation of the color.
Manufacturers can use a single pigment or combinations of multiple pigments to produce a wide gamut of colored ceramic tiles. For example, mixing iron oxide and chromium oxide pigments in different ratios can produce olive green colored tiles.
Mixing and Milling
Once the basic raw materials and pigments have been selected, the next step is mixing and milling.
First, the clay, pigments and other powders are added in specific proportions into a batch mixer. This produces a homogeneous mixture of the raw materials.
Next, the mixture is fed into a ball mill which grinds the particles and mixes them intimately together. The fine grinding ensures uniform dispersion of the pigment particles throughout the tile body.
Well-dispersed pigments lead to more consistent coloring and reduced variation in shade across tiles. The commonly used mills for ceramic tile manufacturing include roller mills, attrition mills, and vibrating ball mills.
Forming the Tile
The next stage in manufacturing is forming the tile body into the desired shape. There are various forming methods used:
| Forming Process | Description |
|---|---|
| Extrusion | Forcing tile body through a die to produce a long strip which is cut into tiles |
| Pressing | Compressing tile body in a mold to produce the tile shape |
| Rolling | Using rollers to flatten clay tile body into a sheet which is cut into tiles |
| Dry pressing | Compacting tile body powder in a mold at high pressures |
The colored tile body maintains its color during the forming process. The method chosen depends on the required tile size, thickness, and design.
Drying and Firing
After forming, the ceramic tiles contain around 15-20% moisture which needs to be removed prior to firing. Tiles are dried slowly to remove the moisture without cracking the tiles.
Next, the dried tiles undergo a high temperature firing process which consolidates the body and develops the final color. The firing process involves:
- Heating tiles up to around 1200°C in a kiln.
- Soaking at peak temperature to complete reactions between pigments and clay.
- Cooling the tiles slowly back to room temperature.
During firing, ceramic pigments undergo chemical reactions that produce color in the finished tiles. For example, cobalt oxide forms cobalt aluminate crystals which give the blue color.
The peak firing temperature and kiln atmosphere impact the final tile color. After firing, the tiles may be glazed or polished further.
Secondary Coloring Processes
In addition to coloring the tile body, some secondary processes can also be used to add color effects on tiles:
- Glazing – Applying a colored glass-like coating on the tile surface
- Digital printing – Printing decorative color designs on tile surface using inkjet technology
- Color flashing – Coating tile surface with colored clays which get fused during firing
These methods create unique patterns, textures, and designs on the ceramic tile surface. However, the base tile color is still achieved through pigments in the tile body.
Quality Control
To ensure color consistency across tile batches, strict quality control and testing procedures are followed:
- Tile color is measured using reflectance spectrophotometers and compared to standard references.
- Instrumental measurement is supplemented with visual inspection under standardized lighting conditions.
- Tight control is kept over weighing and mixing of raw materials and pigments.
- Kiln temperatures are closely monitored with multiple thermocouples.
- Fired tiles are sorted to minimize color variation.
These controls allow ceramic tile manufacturers to reproduce colors with great accuracy over long production runs.
Conclusion
The coloring of ceramic tiles is a complex multi-step process. By careful selection and blending of naturally colored clays and inorganic pigments, a diverse range of colors can be achieved in ceramic tiles. The color is developed in the tile body during the firing process and can be further enhanced by secondary decoration methods. With stringent quality control, tile manufacturers can precisely control the color of their products. So the next time you walk into a tile showroom, remember the scientific wizardry behind those vivid colored tiles!