What Kind Of Clay Is Waterproof?

Clay is a type of fine-grained natural material composed primarily of hydrated aluminum silicates. When clay is fired at high temperatures, the molecules bond to become strong, hard and essentially waterproof. However, natural or air-dried clay is inherently porous and permeable to water. Waterproofing clay is the process of treating or modifying clay to prevent water absorption and penetration. This allows clay items to be submerged or exposed to wet environments without damage.

This article will examine the properties that make some types of clay naturally waterproof, methods for waterproofing common clays like kaolin and ball clay, as well as other options for waterproofing clay items. The aim is to provide crafters, artists, and industry professionals with an overview of how to achieve waterproof results when working with clay.

Properties of Clay

Clay has a variety of unique properties that make it a valuable material for pottery and ceramics. Its composition, plasticity, firing temperature, porosity, shrinkage, and different clay types all contribute to its usefulness.

The primary constituents of clay are finely divided minerals such as kaolinite, montmorillonite-smectite, illite, and chlorite. Clays also contain quartz, feldspar, calcite, dolomite, and other accessory minerals. The mineral composition affects plasticity, firing properties, color, and more.

Plasticity refers to a clay’s ability to be shaped and molded without cracking or crumbling. Clay becomes plastic and workable when mixed with water. Plasticity results from the sheet-like structure of clay particles and their electronegative surface charge.

Clays fire or undergo permanent hardening at different temperatures depending on composition. Earthenware clays vitrify around 1100°C, stonewares around 1200°C, and porcelains around 1300°C. Firing leads to sintering, densification, and strengthening of the clay.

Porosity describes the volume of void spaces in a clay body after firing. Porosity depends on factors like particle size distribution and firing temperature. Less porous clays are stronger and more watertight.

As clay loses water during drying and firing, it shrinks in volume. Shrinkage rate varies among clay types but is predictable for a given clay body.

Major types of clays include kaolin, ball clay, fire clay, stoneware, earthenware, and porcelain. Each has unique properties suitable for different applications.

Making Clay Waterproof

There are a few main techniques used to make clay waterproof:

Sealing

Applying a sealant to finished clay items creates a barrier that prevents water absorption. Common sealants include polyurethane, epoxy resin, and wax. Multiple coats are usually required for full waterproofing. Sealants can alter the look and feel of the clay surface.

Glazing

Glazes provide a glassy coating that fills pores and prevents water penetration. Glazes must undergo high temperature firing to fuse with the clay body. Multiple glaze layers are typically required for waterproofing. Glazing allows retention of the natural clay surface appearance.

Firing Temperature

Firing clay to higher temperatures causes sintering which fuses clay particles together, reducing porosity and permeability. Tightly packed particles make it harder for water to penetrate the clay. Temperatures of at least 2,200°F are generally required to vitrify clay bodies for water resistance.

Additives

Mixing hydrophobic additives into the clay body can reduce its affinity for water. Common water-repellent additives include silane, wax, silicon oil, and acrylics. These create a molecular barrier within the clay matrix. The quantity of additive must be optimized to balance workability and water resistance.

Naturally Waterproof Clays

Certain types of clay are naturally waterproof due to their mineral composition and firing process. The main naturally waterproof clays are:

Kaolin Clay

Kaolin clay, also known as China clay, is a white, fine clay that is composed mainly of kaolinite. It is formed by the weathering of aluminium silicate minerals like feldspar through a process called kaolinization. Kaolin clay has a high fusion temperature and becomes vitrified when fired at high temperatures, making it impervious to water.

Ball Clay

Ball clay is a secondary clay made up primarily of kaolinite and silicon dioxide. It is sedimentary in origin and has a high plasticity. When fired, ball clay vitrifies and becomes impermeable to water. It is often used in ceramics for making porcelain.

Stoneware

Stoneware refers to pottery that has been fired at high temperatures of around 1200°C to 1300°C. The high firing temperature causes the clay to become vitrified, hard, and non-porous. This makes stoneware extremely water resistant and durable.

Porcelain

Porcelain is a ceramic material made by firing clay, feldspar and quartz at very high temperatures. The composition and firing process results in a non-porous, vitreous ceramic that is impermeable to water. Porcelain’s waterproof properties make it ideal for plumbing fixtures, tableware, and electrical insulators.

Waterproofing Kaolin Clay

Kaolin clay, also known as China clay, is one of the most common clays used in ceramics and pottery. On its own, kaolin clay is porous and permeable to water. However, it can be made waterproof through proper firing and glazing techniques.

When firing kaolin clay, it is important to reach temperatures between 2,372°F and 2,552°F (1,300°C and 1,400°C). At these high temperatures, the clay undergoes vitrification, which fuses the clay particles together, closing the pores and creating a non-porous, impermeable material.

Glazing is another essential step for waterproofing kaolin clay. Glazes provide a glassy coating that seals the surface of the clay. The glaze fills in any remaining pores and creates a water-tight finish. Glazes for stoneware and porcelain clays usually contain silica, flux, and alumina. These ingredients melt during firing to form the impervious glass surface.

With the right firing temperature and glazing, kaolin clay can become fully vitrified and waterproof. Pieces made from kaolin clay are commonly used to make ceramic containers, tiles, sinks, bathtubs and other items that need to hold water.

Waterproofing Ball Clay

Ball clay is a very fine grained and plastic clay that is not naturally waterproof. However, ball clay can be made waterproof through proper sealing and high firing temperatures. When preparing a clay body with ball clay, it’s important to add 10-15% of a fluxing agent like feldspar to lower the maturation point of ball clay. Silica in forms like quartz and flint should also be added to the clay body.

Once shaped, ball clay must be thoroughly dried to remove all moisture. Pieces should be fired slowly up to around 2,200°F to fully vitrify the clay. At these high temperatures, the fluxing agent will allow the silica and alumina in the ball clay to fuse together forming a non-porous material that water cannot penetrate. Care must be taken to reach the optimal firing temperature – if underfired, the ball clay will remain porous and prone to water absorption. But overfiring can lead to deformities. When properly sealed and fired, ball clay can become a durable and waterproof ceramic material suitable for indoor and outdoor use.

Waterproofing Stoneware

Stoneware is a type of clay that becomes vitrified and nonporous during the firing process. This makes stoneware naturally water-resistant. However, stoneware’s porosity can be further reduced through glazing.

Glazing stoneware involves coating the surface with a liquid glass-like mixture before firing. As the stoneware clay vitrifies in the kiln, the glaze melts and fuses with the clay body, creating an impervious glassy coating.

A clear glaze can be used to seal the stoneware while maintaining the look and feel of the bare clay. Colored glazes add brilliant hues while also waterproofing the stoneware. Multiple glaze layers and techniques like wax-resist can create decorative patterns and designs.

Fully vitrified and properly glazed stoneware becomes entirely nonporous. Liquids are unable to penetrate the fused glassy surface layer. This makes glazed stoneware ideal for items like mugs, bowls, vases and more which need to hold water.

Waterproofing Porcelain

Porcelain clay achieves its waterproof properties through high firing temperatures. During the firing process, materials flux and vitrify, fusing the clay particles together to reduce porosity. Typical firing temperatures range between 2,200°F to 2,460°F. At these high heats, the silica and alumina compounds combine, eliminating pores within the clay body to create non-porous ceramics.

Porcelain clays have higher kaolin content compared to other clays, resulting in low shrinkage and reduced warping. The lack of pores prevents water from being absorbed into the material. As such, porcelain products like bathroom sinks, toilets, tiles and cookware do not require additional seals or glazes to achieve water resistance. The dense, non-porous nature of porcelain makes it naturally waterproof.

Other Waterproofing Options

Beyond choosing a naturally waterproof clay or altering the clay’s composition, there are some additional options for waterproofing clay surfaces and objects:

Wax Seals

Applying a wax sealant is one of the simplest ways to make clay waterproof. Beeswax, paraffin wax, and carnauba wax can all be effective. The wax fills in the pores of the clay to create a water-repellent barrier. Wax seals need to be reapplied regularly as they wear off over time.

Commercial Waterproofers

There are specialty waterproofing products made for ceramics and clay. These often contain silicone, acrylics, or other water-resistant compounds. When applied to the bisqued or finished surface, they soak in and seal the pores. Commercial waterproofers are convenient and long-lasting but can be more expensive than homemade options.

Conclusion

In conclusion, there are several types of natural clay that are waterproof like kaolin, ball clay, stoneware, and porcelain. Kaolin clay in particular has a naturally fine particle size and plasticity that makes it water-resistant. However, other clays can also be made waterproof through firing at high temperatures or by adding other materials like silicone or sodium silicate. When fired between 1200°C and 1400°C, many clays vitrify which seals the pores and creates a non-porous ceramic that water cannot penetrate. Waterproofing is important for many applications like pottery, tiles, sanitaryware, and outdoor construction where water exposure is expected. The best options for waterproof clay depend on the project requirements and production methods. With an understanding of clay composition and waterproofing techniques, high quality water-resistant clay products can be achieved.