How Do You Permanently Harden Clay?

Clay is a soft, malleable substance made from minerals and water. It is a popular material for artists and crafters to shape into pottery, sculptures, tiles, and more. However, clay in its natural soft state is fragile and permeable. Permanently hardening clay is necessary in order to strengthen it and help clay creations last without deforming.

There are several methods that can be used to permanently harden clay to make finished pieces sturdy and durable. The most common techniques involve exposing the clay to air, heat, or chemical sealants that facilitate hardening. The method chosen will depend on factors like the type of clay, size of the piece, desired hardness level, time constraints, and available equipment. This article will provide an overview of the major techniques for permanently hardening clay.

Air Drying

Air drying is one of the simplest ways to harden clay permanently. It involves shaping moist clay and allowing it to sit at room temperature until all the water evaporates. As the water evaporates, the clay particles bond together, forming strong intermolecular bonds that harden the clay.

Air drying takes advantage of the natural hardening process and doesn’t require any additional equipment or materials. It is a slow process, often taking a week or longer for pieces to fully dry and harden. The length of drying time depends on factors like the thickness of the clay, the climate, and air circulation.

Air drying is best for simple, small clay pieces and those that won’t be exposed to water. The slow drying time increases the chances of cracking or warping. Air dried pieces may also absorb moisture over time, becoming less hard. Still, air drying is an accessible option for hobbyists or classrooms looking to harden clay without additional steps.

Oven Baking

Baking polymer clay in a regular kitchen oven is a common way to permanently harden it. The ideal baking temperature for clay is generally between 275°F and 300°F. At these temperatures, the polymers crosslink to cure and harden the clay. Bake times depend on the clay thickness and brand, but tend to range from 15 minutes for thin pieces up to 1-2 hours for thicker sculptures.

There are several advantages to oven baking polymer clay:

• It’s accessible, since most kitchens already have an oven.
• You can bake multiple pieces at once.
• Oven baking evenly heats the entire sculpture.

Some downsides include:

• The process takes more time than other methods.
• Oven temperatures may fluctuate, leading to under- or over-baking.
• There’s higher electricity usage.

Oven baking works best for pieces under 1 inch thick. Thicker pieces may end up with cracks or uncured centers after baking. Oven baking can be used for most any clay sculpture, but works especially well for detailed pieces needing an even, thorough cure.

Microwaving the Clay

Microwaving is a convenient and fast way to harden modeling clay and polymer clay. It typically takes 1-5 minutes depending on the thickness and amount of clay. Be sure not to microwave clay too long as it can burn or scorch.

To microwave clay, form it into the desired shape first. Place the clay sculpture on a microwave-safe plate or dish. Microwave it on high, checking it every 30 seconds until hardened. The clay may still be flexible when removed but will firm up as it cools.

Pros of microwaving clay:

• Very fast, takes only a few minutes
• Easy, requires no additional tools or materials besides a microwave
• Good for small pieces and thin clay

Cons of microwaving clay:

• Can unevenly heat thicker pieces
• Risk of scorching or burning the clay if microwaved too long
• Not recommended for baking polymer clay

Microwaving is ideal for quickly hardening thin pieces of modeling clay. It’s not the best method for baking detailed polymer clay sculptures since it may not fully cure the clay. Take care not to overheat the clay when microwaving.

Freezing

Freezing is another technique that can be used to permanently harden clay projects. Here’s an overview of the freezing process and its pros and cons:

To freeze clay, place the finished clay piece in a freezer safe container or bag. Allow ample room for the clay to expand as it freezes. Seal the container and place in the freezer overnight or for at least 24 hours. Once frozen, remove the clay and allow it to fully thaw. This cycle of freezing and thawing will cause the clay to harden permanently.

Pros of freezing clay:

• Simple process using common household equipment
• Energy efficient
• Allows time for clay to fully harden

Cons of freezing:

• Can cause clay to become brittle
• Risk of cracking or distorting shape
• Requires significant time for freezing/thawing

Freezing works best for smaller, thinner clay pieces where distortion is less likely. It may not be ideal for larger or more intricate sculptures. Test on samples first before freezing irreplaceable clay artworks.

Using Commercial Sealants

Types of Sealants

There are different types of commercial sealants that can be used to permanently harden air-dry clay:

• Acrylic seals – These are water-based acrylic polymer sealants that penetrate the clay and act as a flexible plastic barrier.
• Polyurethane seals – These are made from polyurethane resin and work by coating the clay in a protective plastic layer.
• Epoxy resin – Epoxy is used to fully saturate the clay and binds everything together to harden it.

Application

Sealants and resins are applied by brushing or spraying a thin coat over the entire surface of the dried clay artwork. 2-3 coats are usually needed for full protection and hardness. It’s important to apply the sealant or resin evenly according to the manufacturer’s directions. The clay must be fully cured before applying any sealant.

Pros and Cons

Pros:

• Provides a glossy, waterproof, and durable finish.
• Easy to apply with basic equipment and no special skills needed.
• Acrylics and polyurethanes are non-toxic when dry.
• Low cost options available.

Cons:

• Can yellow or darken over time, especially with exposure to UV light.
• Surface can become tacky in humid environments.
• Brushing on sealant can soften clay details.
• Epoxy resins require careful safety precautions when applying.

Using Polyurethane or Resin

Polyurethanes are a common way to permanently harden clay sculptures or pottery. There are a few types of polyurethane that work well:

• Two-part polyurethane resins – These come as separate resin and hardener components that you mix together before applying to the clay surface.
• UV-curable polyurethane – These resins cure and harden when exposed to UV light, so require a UV lamp.
• Polyurethane sprays – These are spray-on polyurethane coatings that dry to a hard protective finish.

To apply the polyurethane, make sure the clay is completely dry first. Apply the resin carefully according to the product instructions, using a brush to get an even coating over the entire surface. For two-part resins, thoroughly mix the resin and hardener before starting. Spray-on polyurethanes are easiest to apply evenly. No matter the type, apply the polyurethane in a well-ventilated area and allow ample time to fully cure and harden before handling the clay.

Polyurethane produces a high-gloss, durable, plastic-like finish over clay. It permanently hardens the sculpture so it does not deform or scratch easily. The coating also helps protect the clay from moisture and prevents colors from fading over time. Compared to other sealants, polyurethane forms the strongest protective barrier for long-lasting results.

On the downside, polyurethane can leave brush stroke marks if not applied carefully and evenly. The fumes and drying process also take several hours, so polyurethanes require more patience than quick commercial sealants. Still, for professional-quality, permanent hardness of clay, polyurethanes offer a robust and lasting solution.

Heating in a Kiln

Kilns used for hardening clay reach temperatures between 1,800-2,400°F. Heating clay items in a kiln chemically changes them through vitrification. This fuses the silica and other minerals within the clay, transforming them into glass-like substances.

The pros of using a kiln include:

• Produces extremely hard, durable ceramic finish on clay items
• Allows precise temperature control for consistent results
• Kiln-fired ceramics are food safe, waterproof, and scratch resistant

The cons of kiln firing include:

• Requires expensive, specialized equipment (kiln)
• Industrial kilns use a lot of energy and produce carbon emissions
• Individual results may vary for beginner hobbyists

Kiln firing is best for:

• Creating durable products like ceramic dishware, pots, or sculptures
• Artists or artisans hardening their clay art for exhibition or sale

Choosing a Method

When selecting the best hardening method for your clay project, consider the following factors:

• Desired permanence – Some methods like air drying are reversible, while others like commercial sealants create a permanent hard finish.
• Type of clay – Porous clays absorb more liquid so dry slower than dense clays. Sculptural clays need more care when baking to avoid cracking.
• Facilities and tools – Oven baking needs consistent home oven access. Kiln firing requires a high-temperature kiln.
• Time available – Air drying can take weeks vs. minutes in a kiln. Faster drying can cause cracks in larger pieces.
• Intended purpose – Decorative items may need only an air dried finish, while functional pieces like mugs require waterproof methods.
• Desired appearance – Glossy commercial finishes differ from matte ones. Unsealed terra cotta has an organic, earthy look.
• Safety issues – Heat methods can burn skin if handled soon after. Chemical sealants require ventilation during application.

Test small pieces when exploring a new hardening technique. Record results to determine the method that best suits your project goals, timeline, and environment.

Conclusion

There are several ways to permanently harden clay, each with their own benefits and drawbacks:

• Air drying is the most natural method that doesn’t require any chemicals or special equipment but takes a long time.
• Oven baking produces a hardened clay much more quickly but requires an oven that goes to the proper temperature.
• Commercial sealants permanently harden clay through a chemical reaction but can be toxic. Using polyurethane or resin works similarly.
• Heating clay in a kiln yields the strongest, hardest results but requires large specialized equipment.

When choosing a hardening method, consider the desired hardness level, available time and equipment, toxicity concerns, and final use of the clay piece. With an understanding of these key factors, you can select the permanent clay hardening technique that fits your needs.