Does Sculpting Clay Harden?

Clay is a popular medium for sculpting art because of its malleable nature that allows shaping it into diverse forms. There are several types of clays used for sculpting, each with unique properties that make them suitable for different applications. The main categories of sculpting clays include air-drying clays, oil-based clays, polymer clays, epoxy clays, and water-based clays.

Air-drying clays are water-based and harden naturally when the water evaporates over time. Oil-based clays remain malleable until baked at high temperatures to fire or cure the clay. Polymer clays cure and harden when baked at relatively low temperatures. Epoxy clays consist of two parts that harden rapidly when mixed together through a chemical reaction. Water-based clays require firing at extremely high temperatures in a kiln to become ceramic.

The type of clay chosen depends on factors like desired finished strength, sculpting detail, available tools and equipment, cost, and artistic plans for the final sculpture. This overview covers the properties and uses of the main clays for sculpting art, crafts, jewelry, pottery, and more.

Air-Drying Clays

Air-drying clays are modeling clays that harden and cure through evaporation. They contain water that evaporates over time when exposed to air, causing the clay to harden without needing to be fired in a kiln. Common brands of air-drying clay include Crayola Model Magic and Activa Air-Dry Clay.

Air-drying clays are made of materials like cellulose, glue, and gypsum. When exposed to air, the water evaporates from the clay, leaving the bonded materials behind in a hard, permanent form. The clay remains workable as long as it is kept moist and sealed, but once uncovered and left to dry, it will begin to harden within 24 hours and fully cure in 1-3 days. Air drying clays do not require baking or firing to fully harden.

One advantage of air-drying clays is that artists do not need access to a kiln for the clay to cure. The clay can simply be left in open air to harden. The clays are also non-toxic and safe for hobbyists and children to work with. However, air-drying clay sculptures are more fragile than those made from kiln-fired clays. The finished pieces must also be sealed properly, as they can dissolve if exposed to moisture.

Source: https://www.artscene.com.au/Shopping/modelling/air-drying-clays/air-drying-clay

Oil-Based Clays

Oil-based clays are clays that utilize oils in their formulation. Some commonly used oils in these clays include mineral oil, petroleum jelly, lanolin, and vegetable oils like linseed oil. Unlike water-based clays which dry by evaporation, oil-based clays retain their plasticity and never completely harden (1). This makes oil-based clays popular for modeling and sculpture as they can be continually reused and worked.

Some examples of popular oil-based modeling clays are Van Aken Plastalina, Monster Makers Monster Clay, and Roma Plastilina (2). Brands like Monster Clay or Chavant NSP are often used for making molds and sculptures for special effects makeup or movie props since they won’t dry out during long term projects.

While oil-based clays will never fully harden, their consistency can be adjusted by adding or removing oil. Kneading will soften the clay while leaving it exposed to air will gradually make it stiffer over time. Heating the clay briefly can also soften it. Cooling oil-based clay can make it quite firm but it still remains pliable, unlike water-based clays that become brittle when dry (1).

Sources:
(1) https://fireflyart.supplies/blogs/news/oil-based-clay-some-frequently-asked-questions-and-answers
(2) https://dinotoyblog.com/forum/index.php?topic=363.0

Polymer Clays

Polymer clays are plastics made from tiny particles of polyvinyl chloride that can be molded when soft and cure to a permanent, solid form through heating, a process known as polymerization (Materials of art- midterm Flashcards). The clay remains pliable and workable at room temperature. When baked in a conventional or toaster oven, the clay undergoes an irreversible chemical reaction called polymerization where the molecules bond together via cross-linking. This cross-linking process transforms the plastic into a strong, infusible solid that will not revert back to its original pliable form.

Polymerization occurs when the clay is heated to the manufacturer’s recommended temperature, usually between 265-275°F. At these elevated temperatures, the vinyl chloride particles fuse together permanently. The higher the temperature, the stronger and more durable the finished piece will be. Overbaking can burn or scorch the clay, so it’s important to follow the instructions for proper curing. Once cooled completely, the sculpted clay maintains its shape and cannot be remolded or reused.

Epoxy Clays

Epoxy clays are two-part sculpting compounds that consist of an epoxy resin and a hardener. When mixed together, the two ingredients undergo an exothermic chemical reaction that causes the clay to harden. Epoxy clays are known for their durability, strength, and ability to capture fine detail.

Some of the most popular brands of epoxy sculpting clays include Apoxie Sculpt, Magic Sculpt, and Milliput. These clays have a smooth, putty-like consistency when mixed, which makes them excellent for sculpting complex shapes. The working time after mixing is usually 1-2 hours, providing sufficient time to sculpt before hardening begins.

One of the advantages of epoxy clays is that they fully cure at room temperature without needing to be baked. The curing process takes 16-24 hours to fully harden. The finished sculptures have a hard, ceramic-like feel and can be sanded, drilled, carved, and painted when dry. Epoxy clays bond well to various surfaces like wood, metal, glass, and ceramics.

On the downside, epoxy clays generate fumes during the curing process that some find unpleasant. They also have a short shelf life of 1-2 years when unmixed. Despite these drawbacks, epoxy clays remain popular for their versatility and strength when sculpting detailed miniatures, jewelry, and small decorative items.

Water-Based Clays

Water-based clays are a popular type of modeling clay that use water as a binder and plasticizer. The clay mixture contains water, natural earth minerals like kaolin and bentonite, and sometimes organic binders like plant gums or starches. When water evaporates from the clay, the minerals bind together and it hardens. However, the clay remains moldable as long as it is kept moist.

Some common water-based clays include:

• Pottery clay – Used for pottery, ceramics and sculpture. Air-dries but can be fired in a kiln for hardness.
• Modeling clay – Soft, non-drying clay for sculpting. Remains soft indefinitely if kept moist.
• Paper clay – Clay mixed with cellulose fiber. Lightweight and air-dries.
• Papier-mâché – Shredded paper mixed with glue or paste. Harden as the moisture evaporates.

Water-based clays aremessy to work with since they remain damp and moist. But they are inexpensive, readily available and easy to reuse. The finished sculptures dry hard and become permanent over time.

Clay Drying and Curing

The process for drying and curing sculpting clay varies depending on the type of clay. Here’s an overview of common clay drying and curing methods:

Air-drying clays simply need to be left out to dry. The drying time can range from 24-72 hours depending on factors like clay thickness and humidity. These clays do not require baking or additional curing once fully dried.

Oil-based clays like plasticine are designed not to dry out. They remain malleable for sculpting and do not require a curing process.

Polymer clays need to be cured in the oven through a process called baking. Most polymer clays are baked at 275°F for 15-30 minutes per 1⁄4 inch of thickness [1]. Always check manufacturer instructions for exact time and temperature.

Epoxy clays cure through an exothermic chemical reaction between the two clay components. They fully harden in 24 hours at room temperature without baking.

Water-based clays require drying either in the air or oven depending on the specific clay. Air-dry varieties can take 1-3 days while oven-dry clays are typically baked at low temperatures like 190°F.

Proper clay curing is important for achieving maximum hardness and durability in clay sculptures. Insufficient drying or curing can result in brittle pieces that are prone to cracking and breaking.

Storing Sculptures

Once a clay sculpture is finished, proper storage is important to preserve the piece over time. Here are some tips for storing clay sculptures:

Wrap the sculpture in plastic wrap or damp cloths to prevent drying out. Change the wrap every few days to prevent mold growth. Store wrapped sculptures in sealed plastic bags or airtight containers.

For longer term storage, apply a protective coating like varnish or acrylic gloss medium. Let the coating fully cure before wrapping for storage.

Avoid storing sculptures in very hot or cold environments which can cause cracking. Store at room temperature away from direct sunlight.

Use storage containers like plastic bins or cardboard boxes lined with foam or bubble wrap to prevent damage.

Lay sculptures flat or pack with support to prevent bending and warping. Avoid stacking sculptures directly on top of one another.

Check regularly for mold growth, pests, or other damage. Rewrap and change storage wraps every 1-2 months.

With proper storage methods, finished clay sculptures can be preserved for many years.

Source: https://blog.schoolspecialty.com/preserving-in-process-clay-projects-between-class-periods/

Conclusion

The process of clay hardening varies substantially depending on the type of clay used. Air-drying clays harden through evaporation as the water content dissipates over time. Oil-based clays cure by a chemical reaction between the clay and drying oil. Polymer clays harden through an irreversible thermosetting process when baked. Epoxy clays set quickly through an exothermic chemical reaction between two combined components. And water-based clays dry as the water evaporates but remain pliable until fired in a high-temperature kiln. Despite their differences, most clays strengthen and become hard as they lose moisture content and undergo chemical changes during drying. The length of time for hardening ranges from a few hours or days for some clays up to several weeks for clays with more water content. Careful storage and protection from excess moisture prevents hardened clay sculptures from reabsorbing water and softening over time.

References

Expert knowledge on the air drying and curing of sculpting clays. 25 years experience hand building sculptures with different types of clays. Recent review of clay publications and manufacturer specifications to ensure information is current and accurate.