What Makes Clay Into A Waterproof Surface?

Clay is a naturally occurring earthy material made up of fine-grained minerals such as silica and alumina. When mixed with water, clay becomes highly moldable and is used to create various pottery and ceramic objects. However, regular fired clay is quite porous and permeable to water. Making clay waterproof requires additional steps to seal the surface and fill in the tiny pores.

There are several methods that can be used to make clay impervious to water. Applying special water-repellent glazes, wax coatings, resins, and oils during the manufacturing process creates a protective barrier that prevents moisture absorption. Proper maintenance and reapplication of waterproofing agents is also important for long-term water resistance.

This article will provide an overview of how firing, glazes, waxes, additives, and sealers can transform porous clay into a waterproof, non-porous material.

Composition of Clay

Clay is composed of very fine-grained minerals called clay minerals. The most common clay minerals include kaolinite, montmorillonite-smectite, illite, and chlorite. Clay minerals are formed over long periods of time from the chemical weathering of rocks. They are characterized by sheet-like structures and very small particle sizes, typically less than 2 micrometers.

The small particle size and sheet-like structure of clay minerals give clay its unique properties of plasticity and cohesiveness. When mixed with water, the clay particles are able to slide over one another while still sticking together, allowing clay to be molded into different shapes. The clay remains cohesive and maintains its shape after molding due to the electrostatic forces between the charged clay particles.

Clay’s plasticity allows it to be shaped, while its cohesiveness allows it to retain its shape once formed. These properties make clay an ideal material for crafting durable ceramic and pottery objects. The moldability and durability of the final ceramic or pottery object depends significantly on the composition and particle sizes of the clays used. A higher plasticity clay will typically produce more workable and tougher finished wares.

Firing Clay

One of the key steps in creating a waterproof clay surface is firing the clay. Firing refers to heating the clay to high temperatures in a kiln, which causes chemical changes to the clay that are important for water resistance.

At high firing temperatures, a process called vitrification occurs. This is where the silica and alumina in the clay melt and fuse together, creating a dense, non-porous material, similar to glass. As the clay vitrifies, the microscopic pores within the clay body begin to close up and seal. This vitrification process is critical for creating a waterproof surface.

Firing temperatures typically need to reach at least 2,200°F (1,200°C) for vitrification to properly occur in most clay bodies. When fired at lower temperatures, the clay will not fully vitrify and some pores may remain open, allowing water to seep in. Proper vitrification requires high heat in order to seal the clay surface.

In addition to vitrification, firing also chemically hardens the clay through sintering. This is where clay particles partially melt and fuse together for added strength and reduced porosity. Between vitrification and sintering, firing transforms soft, porous clay into a dense, water-resistant ceramic material.

Glazes

Glazes are vitreous coatings applied to pottery before firing. They fuse to the clay body during firing and provide an impervious surface that helps seal the pores of the clay. There are several types of glazes used to make clay waterproof:

Clear glazes contain ingredients like silica, alumina, feldspar, and kaolin that form a hard, transparent layer. They allow the natural color and texture of the clay body to show through. Clear glazes are excellent at sealing the surface and preventing water absorption.

Colored glazes also contain clay, silica and fluxing compounds along with metallic oxides that provide vibrant colors. The pigments make the glaze opaque. Popular colored glaze finishes include celadon, copper red and tenmoku.

Crystalline glazes have compounds that crystallize out during firing to create crystal formations on the surface. They provide a shimmery look along with water resistance.

By filling in the pores and sealing the clay surface, glazes of all types create a waterproof barrier and prevent moisture penetration into the clay body underneath. They are an integral part of making pottery water-resistant.

Wax Coatings

Wax coatings can make clay water-resistant by sealing the surface. Popular options include beeswax and paraffin wax. When applied to the clay surface and buffed, the wax fills pores and creates a protective barrier.

Beeswax has been used for centuries to waterproof objects and surfaces. It provides a natural, non-toxic coating. The wax must be melted prior to application. Multiple thin layers are recommended, allowing each coat to fully dry before adding the next. Beeswax darkens and enhances the natural clay color.

Paraffin wax derives from petroleum. It has a higher melting point than beeswax, so it may provide a more durable coating for items exposed to heat or sun. Paraffin wax goes on white but dries clear. As with beeswax, apply thin layers and allow time to cure between coats.

For best results, ensure the clay surface is completely clean and dry first. Apply wax while the clay is slightly warm to the touch. Use a soft cloth to rub the melted wax thoroughly over the surface, then buff with a clean cloth once dry. Reapply wax periodically to maintain the protective seal.

Water-Repellent Additives

Certain chemicals can be added to clay or glazes that help make the surface water-resistant. Some common options include:

Silicone

Adding a small amount of silicone to glazes creates a water-repellent layer. The silicone rises to the surface during firing. This helps prevent water absorption while still allowing vapor transmission. Silicone is effective at making surfaces like tiles more water-resistant.

Stearates

Metallic stearates are fatty acid salts that can increase a clay body’s water resistance when used as an additive. The stearates help make the clay denser and less porous. Zinc stearate and aluminum stearate are commonly used options. Stearates improve the clay’s workability as well.

Internal Water Resistance

In addition to surface treatments, clays can be formulated to have internal water resistance. Adding grog, which is crushed up fired clay, makes the body less porous. Certain plasticizers also help prevent water absorption. The goal is to create a tight molecular structure that doesn’t allow water to permeate.

Resin Sealers

Acrylic resin sealers provide an additional protective layer that repels water on clay surfaces. The resin seeps into the pores of the clay, binds to the surface, and forms a flexible, waterproof barrier. Acrylic resin is clear, non-yellowing, and dries to a glossy finish. It can be used for indoor or outdoor applications on unglazed, natural clay finishes.

When applied properly, resin sealers are highly effective at preventing water absorption while still allowing vapor transmission. This prevents damage from moisture while letting the clay “breathe”. Two or more coats are usually recommended for maximum protection. Resin sealers need to be reapplied every few years as they gradually wear away from weathering and abrasion over time. Proper surface preparation and application technique is key to getting an even, long-lasting seal.

Compared to wax, resin sealers are more durable and provide better moisture protection. They also maintain the natural look of unsealed clay. However, resin can give an artificial plastic-like sheen in high gloss finishes. Overall, acrylic resin sealers provide excellent water repellency and protection for both artistic and functional clay surfaces.

Oil Treatments

Oils like linseed oil and tung oil can create a water-resistant barrier when applied to the surface of clay. These drying oils penetrate into the clay’s porous structure and undergo a chemical reaction to form a hydrophobic polymeric coating.

Linseed oil is derived from the seeds of the flax plant. It cures to a hard, solid film that repels water. Multiple coats of linseed oil are usually necessary to achieve good water resistance. The oil must be allowed to fully dry and cure between coats. Curing can take weeks or even months to maximize water repellency.

Tung oil is derived from the nuts of the tung tree. It is comprised of unique hydrophobic compounds that enable tung oil to polymerize into a water-resistant finish. Tung oil cures faster than linseed oil. It provides good moisture protection with fewer coats. However, it is more expensive than linseed oil.

Oils are able to penetrate the porous clay surface in a way that paints and other coatings cannot. This allows the oil to provide water resistance without changing the natural look or feel of the clay. Oils enhance the natural beauty of clay while protecting it from moisture.

Maintenance

Clay surfaces need to be regularly maintained or re-treated in order to keep their waterproofing integrity. Many sealants will begin to degrade or wear off over time, especially with heavy use or exposure to the elements.

It’s generally recommended to reapply sealants every 1-2 years, but this can vary based on the specific product used. Pay attention to any label instructions for reapplication timing and directions.

When reapplying sealants, make sure the surface is clean and dry first. Use a stiff brush and mild detergent to thoroughly clean the surface and rinse off any residue. Allow it to completely dry before adding any new coatings.

Wear gloves when working with sealants as many can irritate skin. Work in a well-ventilated area and avoid breathing in fumes directly. Let each coat fully dry as directed before adding more layers.

Be cautious about applying too much sealer at once, as thick or pooled coatings may end up cracking or peeling. Thin, even layers according to the product instructions is best.

With proper, routine maintenance and re-sealing, clay surfaces can retain their water-resistant qualities for many years.

Conclusion

In summary, there are several effective methods for making clay surfaces waterproof, with the most common being firing, glazing, wax coatings, water-repellent additives, resin sealers, and oil treatments. Properly waterproofing clay is crucial for both functionality and appearance. Water can easily stain, erode, and damage unprotected clay. It can also cause finishes to deteriorate and compromise structural integrity. By utilizing one or more of the techniques discussed, it’s possible to seal and protect clay against the damaging effects of water. This allows clay surfaces to maintain their beauty while withstanding regular contact with moisture. Knowing how to properly waterproof clay enables creating durable, long-lasting clay items for indoor and outdoor use.