What Are The Raw Materials For Clay Roof Tiles?

Clay roof tiles have been used to cover roofs for thousands of years. Unlike other roofing materials like asphalt shingles or metal, clay roof tiles are completely natural and provide unparalleled beauty and durability. Clay tiles are made from clay, water and sometimes other natural additives that is pressed or extruded into shape and then fired at high temperatures in a kiln. This produces a strong, lightweight and weather resistant roofing material that can last over 100 years.

The main benefits of clay tiles are their exceptional longevity, natural beauty, fire resistance, and sustainability. Their longevity comes from the durability of the fired clay material, which resists decay and damage from sun, rain, wind and other elements. Clay tiles add striking visual appeal to any home with their range of shapes, textures and earthy colors. And unlike some synthetic materials, clay tiles are non-combustible and historically performed well in urban fires. Finally, clay is among the most environmentally-friendly roofing options since it’s abundant, reusable, and energy-efficient due to its natural insulating properties.

Clay tile roofs have been found in the archaeological remains of ancient Mesopotamian and Egyptian civilizations dating back thousands of years. By the Roman Era, clay tile roofing had become widespread across Europe and the Mediterranean region. Clay roof tiles saw a revival in 19th century England and continue to grace historic buildings and homes today as a timeless, attractive roofing material.

Clay

Clay is a fine-grained natural material composed primarily of hydrated aluminum silicates, which gives clay its plasticity. The clay crystals consist of stacked tetrahedral silicon–oxygen and octahedral aluminum-oxygen layers. The stacking patterns determine the type of clay. There are three main types of clays used in manufacturing clay roof tiles:

Kaolin – Also known as china clay, kaolin has a low plasticity and high fusion point, making it suitable for porcelain tile manufacturing. It fires to a white color.

Ball clay – Highly plastic clay that fires to a light cream or tan color. Ball clay promotes vitrification at lower temperatures and improves the plasticity and strength of clay bodies.

Common clay – Abundant low-grade clay that fires to red or brown colors. Common clay provides bulk and strength to clay tile bodies.

Shaping and Firing

Clay can be shaped into tiles through a few different methods. The traditional method is to press moist clay into open molds by hand or using basic tools. More modern techniques may involve extruding clay through a die cutter to produce a continuous tile ribbon that gets cut into individual tiles.

Once the wet clay tiles are formed, they need to go through a firing process to permanently harden them. Firing takes place in a special oven called a kiln that heats the tiles to over 1000°C. Several different kiln types exist, including tunnel kilns, intermittent kilns, and roller hearth kilns. Firing clay causes chemical changes that fuse the clay particles together into a strong, stable ceramic.

The time and temperature profile of the kiln firing process influences the characteristics of the final tile product. Fast firing tends to produce a darker, more varied color while slower firing maintains a reddish tone. The firing schedule also impacts tile strength, porosity, and water absorption.

Clay Sources

Clay is abundant across the world, with major deposits located in many countries. Some of the most common places to mine clay include:

• Brazil – The Amazon River Basin contains rich kaolin clay deposits.
• United States – Large clay deposits found in states like Georgia, Tennessee, and South Carolina.
• China – Kao-Ling mountain region is a major source of kaolinite clay.
• India – Clay mines located in states like Rajasthan and Gujarat.
• United Kingdom – Ball clay mined in the southwest England region.
• Germany – Deposits of ball clay found in the Westerwald region.
• Ukraine – Kaolin clay mined near Kryvyi Rih and Zaporizhzhya.
• Australia – Deposits located in southwestern Queensland.

Clay deposits are spread globally across all continents except Antarctica. Countries with substantial clay reserves use it extensively for local brick, tile, and pottery production.

Other Natural Additives

In addition to clay, other natural materials are often added to the clay body to modify the properties and quality of the finished tiles. Two of the most common natural additives are sand and straw.

Sand

Sand particles, typically silica sand, are frequently added to clay tile mixtures. The sand serves several important functions:

• It prevents the tile from shrinking and cracking excessively during drying and firing.
• It helps make the clay mixture more porous, allowing water vapor to escape during drying.
• It improves the structural strength and durability of the finished tile.
• It facilitates even heat distribution during firing.

Sand is added in percentages ranging from 10-30%, with higher amounts typically used for tiles that will undergo very high temperature fast firings.

Straw

Chopped straw is also commonly added to clay tile mixtures. Like sand, straw improves the drying behavior of the clay and prevents excessive shrinkage and cracks. As the straw burns out during firing, it leaves tiny channels in the clay that facilitate water vapor release. Straw also enhances the structural integrity and strength of the final fired tiles. The typical straw content is 3-5% by weight.

Both sand and straw play a critical role in minimizing defects and producing high quality, durable clay roof tiles and other clay products.

Artificial Additives

In addition to naturally occurring materials like sand or crushed rock, clay roof tiles can also contain artificial additives to modify their properties during manufacturing. Two of the most common artificial additives are oxides for adding color, and cement or concrete to increase strength.

Metal oxides are frequently added to clay to produce tiles in different colors. For instance, cobalt oxide produces blue tiles, chromium oxide makes green tiles, and iron oxide creates red roof tiles. These oxides are mixed into the clay body before shaping and firing. The oxides undergo chemical reactions during firing, creating stable colored compounds in the final tile.

Small amounts of Portland cement or concrete may also be blended into the clay. The cement acts as a flux, promoting vitrification and densification during firing. This makes the tiles stronger and more water resistant. Too much cement can negatively impact the tile’s appearance, so only small percentages are typically used. The cement also enables faster drying of the tiles after shaping.

Tile Production Process

The production of clay roof tiles involves several steps from start to finish. Here is an overview of the typical manufacturing process:

Preparation – The raw clay materials are crushed, cleaned and blended according to specifications. Water is added to achieve the desired consistency for shaping.

Molding – The clay mixture is pressed into molds to form the basic tile shape. This can be done by hand, but automated hydraulic presses are more common in modern facilities. Tiles may be extruded through a die cutter for a continuous automated process.

Drying – The molded tiles are dried at room temperature or in climate-controlled driers to remove excess moisture before firing. Drying time ranges from several hours to days depending on tile size.

Firing – The dried tiles are fired in high-temperature kilns exceeding 1,000°C to permanently harden the clay. Firing may take place in tunnel kilns, shuttle kilns or other specialized equipment. The firing process vitrifies the clay into a durable, water-resistant ceramic.

Finishing – After firing, the tiles may be treated with surface coatings for aesthetics and additional weatherproofing. Tiles are also sorted, packaged and shipped from the manufacturing plant.

Automation has allowed modern tile factories to maximize efficiency and output. Computerized mixing stations, conveyor belts, and robotics streamline the various production stages. Quality control systems help maintain consistent standards. However, the basic manufacturing principles remain grounded in the fundamental steps of clay preparation, molding, drying and firing.

Quality Control

Quality control is an essential part of the clay roof tile production process. Tiles must meet certain standards for durability, strength, and water resistance in order to be effective and withstand outdoor conditions over many years.

Durability testing puts finished tiles through simulations of years of weathering and thermal stress. Tiles are subjected to repeated freezing and thawing cycles, then checked for cracks, chips, and other damage. Only tiles that emerge undamaged pass this test.

Strength testing determines the amount of force tiles can withstand before breaking. Tiles are subjected to increasing pressure until they crack or shatter. The force required provides a metric for minimum strength standards. Stronger tiles are less likely to be damaged by impacts, walking, or other forces when installed on a roof.

Water resistance is evaluated by measuring water absorption. Tiles are partially submerged, then weighed before and after to determine how much water penetrated the material. Tiles that absorb less water are more resistant to rain, snow, and moisture damage. Absorption levels below 3% are considered optimal.

By passing a rigorous battery of quality control tests, manufacturers can ensure their clay roof tiles will last for decades under real-world conditions. Only tiles that demonstrate excellent durability, strength, and water resistance make it through production and onto roofs.

Comparisons Between Clay Tiles and Other Roofing Materials

Clay roof tiles have some key differences compared to other common roofing materials like concrete tiles and natural slate:

Clay vs Concrete Tiles

Clay tiles are more porous than concrete, which allows them to breathe better. This reduces the risk of condensation building up under the tiles. Concrete also weighs more than clay, putting more load on the roof structure. On the other hand, concrete is more durable and impact-resistant than clay.

Clay vs Slate

Slate is an extremely durable natural stone roofing material. It’s heavier than clay and more waterproof as well. However, slate is significantly more expensive than clay tiles. Clay offers better impact resistance and can be easier to work with for roof installations and repairs. The color and style options are also greater with clay.

In the end, clay strikes a nice balance between durability, water-resistance, weight, cost, and aesthetics compared to concrete and slate roofs.

Conclusion

Clay roof tiles have been used for centuries due to their durability, aesthetics, and natural water-shedding abilities. While concrete and composite roofing materials provide cheaper and lighter alternatives, clay remains popular for high-end residential and commercial projects. Improvements in tile manufacturing allow for greater precision and consistency while retaining the rustic, natural look.

Looking forward, clay roofing will likely retain its niche in the market. More sustainable production methods and renewable energy for firing kilns can reduce the environmental impact. Customizable shapes, textures, and colors present new design opportunities. Blends with recycled materials may provide a lighter and cheaper product. And clay’s durability ensures roofing installations will last for generations. While the fundamentals remain unchanged, clay roof tile technology continues to incrementally improve while meeting the aesthetic desires of homeowners.