Can You Harden Soft Clay?

ByScarlett Evans

What is Soft Clay?

Soft clay is a type of modeling clay that maintains a soft, malleable texture even when dry. It is made from natural clays and minerals that have not been fired at high temperatures. The clay remains pliable and does not harden when air dries.

Soft clay is used for arts and crafts, clay modeling, and ceramic sculptures. It can be easily shaped by hand without requiring tools. Artists use soft clay to create pottery, decorative items, jewelry, and more. It provides an easy medium to mold into creative designs.

The pliability of the clay allows continuous shaping even as pieces dry. Soft clay enables sculpting fine details and textures. It does not become brittle or crack when air dried, unlike clays that require firing at high temperatures. This makes soft clay ideal for beginners learning ceramic sculpting techniques.

While soft clay never fully hardens, there are ways to strengthen it and improve durability once air dried. Adding solutions like varnish, glue, or acrylic mediums can harden soft clay. There are also clays called “cold porcelain” or “air dry clay” designed to harden more than traditional soft clays.

Drying

One of the most common ways to harden soft clay is through air drying. All clay contains water which allows it to be molded into different shapes. When clay is left out in the open air, the water slowly evaporates over time. As the water evaporates, the clay particles move closer together causing the clay to stiffen and become less malleable. The more water that evaporates, the harder and more durable the clay will become. This process works for clays like pottery clay, polymer clay, cold porcelain, and some air dry clays. The length of the drying time depends on factors like the thickness and moisture content of the clay, the clay type, and the environmental conditions like temperature, humidity, and air circulation. In general, air drying can take anywhere from a few hours to several days. To speed up drying, the clay can be placed near a fan, heater, or dehumidifier. Once fully dried, the clay will maintain its shape and be much harder and less prone to cracking or damage. Air drying uses evaporation to effectively transform soft clay into a hardened state.

Firing

Firing soft clay in a kiln is the process of applying high heat to chemically transform the clay body into a new ceramic material that is permanently hard. Firing utilizes a special oven called a kiln which can heat to over 2,000°F. At these high temperatures, chemical changes occur in the clay body through vitrification. Silica and alumina crystals diffuse and react, fusing and forming glass bonds that lock the structure in place.

The end result of firing is a ceramic material that is impervious to water and incredibly hard and durable compared to unfired clay. Firing temperature and length of time in the kiln have a significant impact on the properties of the finished ceramic. Higher temperatures result in more complete vitrification whereas lower temperatures maintain more porosity. The higher the temperature, the harder, stronger, and less porous the fired clay becomes.

Additives

One way to harden soft clay is by mixing in ceramic additives like grog or porcelain. Grog is ground up pre-fired clay or brick that absorbs moisture from the clay body, making it harder and less sticky when wet. Porcelain, on the other hand, is very fine clay that is already quite hard in its raw form.

When grog or porcelain particles are mixed into a soft clay body, they provide structural support and reduce shrinkage. As the clay dries, the grog/porcelain particles help bind the clay particles together in a rigid matrix. This makes the clay stiffer and less prone to slumping or deforming as it loses moisture.

The more grog or porcelain added, the harder and less plastic the clay becomes. A good rule of thumb is to add no more than 25% grog or porcelain to maintain enough clay plasticity to throw on the wheel or hand build without excessive cracking. However, for clay sculpture that doesn’t require throwing, a higher grog/porcelain content up to 50% can be very effective for stiffening the clay.

Polymer Clay

Polymer clays, also called plasticine-like clays, are a popular type of modeling clay. They were first developed as a clay substitute in the 1930s. Polymer clays are a type of clay that can be hardened without a firing process. The material hardens into a solid form when heated in a household oven for 15-30 minutes at around 230-300°F (110-150°C). The most popular brands of polymer clay include Fimo, Sculpey, Kato Polyclay, Cernit, and Formello.

Polymer clays harden through a curing process called polymerization. The clays contain polymers such as polyvinyl chloride (PVC) and plasticizers. When heated at the right temperature, the polymers crosslink with each other, forming stronger bonds. This transforms the soft, pliable clay into a hard, durable solid that keeps its shape. Items made with polymer clay can have fine details and do not weaken or decompose over time like some air-dry clays.

Cold Porcelain

Cold porcelain is a type of soft clay that hardens without needing to be fired. The air drying process allows the clay to set and become solid over time. To create cold porcelain, a mixture of cornstarch, glue, and often baby oil or lemon oil is used. The ingredients are combined to form a malleable dough that has a similar consistency to traditional clay.

As the cold porcelain is exposed to air, the water in the mixture evaporates. Simultaneously, the glue hardens over several hours or days, causing the clay to become rigid. Though it starts out highly flexible and soft, the finished piece will be hard and durable like porcelain. Cold porcelain is lightweight, making finished crafts easy to display.

One of the main advantages of cold porcelain is that no special equipment is required for the drying and hardening process. It does not require heating in a kiln like typical pottery clays. This makes it more accessible for hobbyists and children. The finished pieces also resemble china or bisqueware in their smooth, matte appearance.

Cold porcelain continues drying and hardening indefinitely when stored properly. Pieces made from it can last for many years as decorations, jewelry, or ornaments without chipping or cracking. As an air-dry clay alternative, cold porcelain delivers stunning results minus the firing process.

Air Dry Clay

One popular solution for hardening soft clay is to use air dry clay. This type of clay is
specifically formulated to harden as the water evaporates without needing any additional
treatment.

Air dry clays are made with self-hardening properties that activate when they are
exposed to air for the appropriate amount of time. The water in the material gradually
evaporates and leaves behind a hardened solid shape. Different brands have
slightly different rates of hardening and moisture levels but they all work on this basic
principle of drying out.

Some of the advantages of air dry clays are that they are non-toxic, very affordable and
simple to use. They do not require any expensive equipment, just adequate air exposure
and time to fully harden. The finished pieces will be durable yet can later be soaked
in water to become malleable again for reuse if desired.

Oven Bake Clay

Oven bake clay is a type of modeling clay formulated to harden when baked at relatively low temperatures in a standard home oven. Typically, oven bake clays cure and harden when baked for 10-15 minutes per 1⁄4 inch thickness at temperatures between 130-275°F (55-135°C). Popular brands of oven bake clay include

  • Sculpey
  • FIMO
  • Premo!
  • Cernit
  • Formello

The advantage of oven bake modeling clay is that it can be hardened into durable sculptures using basic kitchen equipment. It does not require a kiln or special tools. The clay remains flexible and workable until baked in an oven, allowing time for reshaping, smoothing, carving, etc. Once cured in the oven, the sculptures become solid and permanent.

Oven bake clays come in a wide range of colors that can be blended and mixed together. Various liquid and powdered products are also available to add color accents, dimensional details, or textured effects. The completed sculptures have an appealing finish resembling porcelain or ceramic.

Storage and Handling

When it comes to storing soft clay before you’re ready to harden and finish the piece, it is important to keep the clay from drying out. Most clays will dry out when exposed to air, but this can be prevented by keeping them wrapped in plastic or sealed in air tight containers when not in use. Here are some of the best ways to store soft clay to keep it soft and workable before hardening:

  • Wrap unused clay tightly in cling wrap/plastic wrap or ziplock bags to prevent air exposure
  • Store blocks or clumps of clay in air tight plastic bins, containers or bags
  • Keep clay stored in areas out of sunlight, heat, and excess ventilation like outdoor sheds or garages as these can speed up drying
  • Use a clay spray or damp paper towels to lightly moisten stored clay to help retain softness
  • Clean tools and surfaces thoroughly so old clay does not dry out and contaminate fresh clay

If clay begins to dry out and harden, it may still be rehydrated and softened in some cases by massaging it until pliable or soaking it in water. However, dried out clay can become brittle and cracked, so it is always best to first practice proper storage and handling of soft clay to prevent loss of quality and workability.

Conclusion

In summary, there are several main methods for hardening soft clays depending on the type of clay and the desired hardness:

  • Air drying – Allowing clays like air dry clay or cold porcelain to dry and harden at room temperature through evaporation.

  • Oven baking – Heating polymer and oven bake clays in the oven to cure and harden them through a chemical reaction.

  • Firing – Exposing clays to very high temperatures in a kiln, which fuses the clay particles together into a hardened state through sintering.

  • Adding hardeners – Mixing in substances like Mod Podge or liquid starch to air dry clays before drying/curing.

The optimal hardening method depends on factors like the clay type, the resources available, the level of hardness desired, the intended application, and more. With various clays and techniques to choose from, hardening soft clays is achievable through planning and proper execution.

Similar Posts