Polymer Clay

Polymer clay is a sculptable material based on the polymer polyvinyl chloride PVC. It usually contains no clay minerals, and is only called “clay” because its texture and working properties resemble those of mineral clay. It is sold in craft, hobby, and art stores, and is used by artists, hobbyists, and children.

Material

Polymer clays of all brands contain a basis of PVC and one or more of several kinds of liquid plasticizer. Pigments may be added to the translucent base to create a variety of colors, along with small amounts of kaolin or white china clay or other opaquing agents where opacity is desired. Mica may also be added to simulate pearlescent and metallic effects.

History

Bakelite, an early plastic used in both practical and decorative applications, was extremely popular with designers and had an early form of polymer clay available in kits, but the phenol base of uncured Bakelite was flammable and these were discontinued. Modern polymer clays are based on a plastic modeling compound brought to the attention of German doll maker Kathe Kruse in the late 1930s as a possible replacement for plastics that had become difficult to obtain during the early days of World War II. It was not suitable for use in her doll factory, so Kruse turned it over to her daughter Maureen, who was known in the family as “Fifi”. The formulation was later sold to Eberhardt Faber and marketed under the name “FIMO” (FIfi’s MOdeling Compound) in honor of Maureen.

Meanwhile, in the early 1940s, Zenith Products Company was founded in Schiller Park, IL. Zenith began as a company that manufactured coatings for the fastener industry: waxes, hot melt compounds, and electrical insulating varnishes. The product that we know today as “Sculpey” was originally formulated for potential use as a thermal transfer compound – to conduct heat away from the cores of electrical transformers. However, this formulation was not successful for that purpose, so the compound was temporarily shelved. A visitor to the manufacturing plant was “doodling” with a lump of the clay-like substance and created a small figure. It was then baked in a lab testing oven – and Sculpey was “discovered” as a sculpture medium. This happened in the mid 1960’s. By 1967, it was being manufactured and sold on a small scale in the United States.

The history of polymer clay as an art medium is only decades long, unlike many media that have been around for centuries and have long traditions. This newness means that there is a great deal of innovation by users of polymer clay. Often, ideas are born by borrowing from the traditions of some other materials, such as metalworking (mokume-gane), ceramics, glass (millefiori, lampwork), paper, etc.

Properties

Original formulations of polymer clay remain soft until cured at relatively low temperatures, but air-dry polymer clays have recently been added to the market. Traditional polymer clay hardens by curing at temperatures created in a typical home oven, generally at 265 to 275 °F (129 to 135 °C), for 15 minutes per 1/4” (6 mm) of thickness, and does not shrink or change texture during the process. When properly conditioned and cured, most clays create items that will not break if dropped or normally stressed. Cured pieces may have additional layers or enhancements added and be re-cured with no ill effect. As long as the maximum curing temperature is not exceeded, there is no limit to the number of times a piece can be re-cured. After it has cured, the clay surface can be left as is, sanded and buffed, or finished with a water-based varnish.

Leading brands of polymer clay include Fimo; Sculpey, Premo and Studio by Sculpey, created by Polyform Products; Cernit; Formello; Modello; and Kato Polyclay, formulated for Van Aken by Donna Kato, an early artist for Fimo. Though the brands differ slightly in properties such as plasticity, translucence, curing temperature, and flexibility when cured, most are suited to a variety of applications. Specialty formulations include clay that remains permanently flexible when cured, eraser clay, and colorless and tinted liquid “clays” that can be used as slips, glazes, and adhesives.

Polymer clay is available in many colors. “Special-effect” colors such as translucent, fluorescent, glow-in-the-dark, mica-containing “pearls” and “metallics,” and variegated “stone” colors containing contrasting fibers are also available. Standard colors, which vary from brand to brand, can be mixed to create a virtually infinite range of custom colors, gradient blends, and other effects. Judith Skinner is credited with inventing a technique that uses a pasta machine to create consistent gradient-blended sheets of color. This technique has many applications and is one of the basic skills developed by hobbyists or artists in the medium.

Polymer clay can be colored with other media. Paint, ink, colored pencil, chalk, metallic or mica-containing powder, metal leaf and foil, glitter, and embossing powder can be applied to the surface. The same materials also can be mixed in as inclusions; this is often done with translucent clay. When acrylic paint is cured onto the surface, it forms a permanent bond with the surface.

Few tools are essential for use with polymer clay, and these can often be found around the house. The most widely used cutting tools are tissue blades, which are extremely thin and sharp, though craft knives and other blades can be used. A pasta machine is often used to create sheets of uniform thickness, to mix colors, to condition the clay, and to create patterned sheets. A “clay gun” or extruder with interchangeable die plates allows creation of lengths of clay in a variety of uniform sizes and shapes. Most molding and modeling tools used by traditional sculptors are suitable for polymer clay, but artists often create improvised cutting, piercing, molding, and texturing tools from items used in sewing, cooking, woodworking, and paper crafts.

Uses and techniques

Polymer clay can be used in many ways, a number of which have been generalized from other art or craft techniques. Some uses for polymer clay include:

• Sculpting. Hand-shaped items can be any size from “miniatures” to quite large. Bas relief can also be created; clay clothing and accessories can be made for sculpted figures.

• Creating beads and jewelry of all kinds, such as pendants, earrings, barrettes, and buttons.

• Covering items made from materials such as glass, metal, cardboard, terra cotta, and some plastics. Popular items for covering include pens, eggshells, votive candle-holders, and switch-plates. Larger items, such as tables, can also be veneered.

• Creating vessels large and small. Jars, boxes, bowls, and container pendants can be created freestanding, or over permanent or removable armatures.

• Creating simulations or fauxs of many natural and fabricated materials such as jade, turquoise, marble, granite, metal, ivory, wood, leather, stained glass, mosaic, and cloisonne.

• Onlaying clay with other materials to create collages.

• Creating paintings with polymer pastes, and bas reliefs.

• Creating practical utility items, such as frames, games and game pieces, dioramas, toys, mini-books, notebook covers, greeting cards, and postcards.

Techniques for working with polymer clay include:

• Gradient (Skinner) blending of two or more colors using sheets of clay and a pasta machine or rolling pin.

• Forming “canes,” which are logs of clay with patterns running through their entire length, from which identical slices can be cut and used in various ways. The patterns created in canes can be simple, complex, or anything in between; they may be pictorial or simply geometric. Canes (and therefore their images) can be “reduced” so that they become quite small, and then combined to make multiple images (millefiori).

• Impressing textures, lines or images into raw clay with rubber stamps, texture sheets, sandpaper, needle tools, or other items.

• Molding: pressing raw clay into molds to create casts and to duplicate textures, shapes, etc. Molds made from metal, glass, rubber, and silicone can be purchased, or custom molds can be made from polymer clay or dedicated molding compounds.

• Extruding clay through shaped die plates to create strands or ropes of uniform size and cross-section.

• “Mokume-gane”: thin slices shaved from distorted stacks of layered clays, powders, and inks and applied to surfaces.

• Using clay to accept “transfers” of images from photographs, drawings, computer-created images or text using a solvent or transfer paper. Images can also be transferred onto freestanding liquid clay films or decals.

• Carving or drilling after the clay has been cured (and backfilled or inlaid, if desired).

• Inlaying cured or uncured clay tiles or chips to create mosaics.

• Multimedia: combining clay with wire, paper, beads, charms, stamps, fabric, etc.

Brand comparison guide

The information provided below is based on the original formulations of these products. All polymer clay brands are undergoing reformulation in response to bans on phthalate esters used as plasticizers, and users have noted dramatically different working properties of the new formulae.

Each brand of polymer clay has inherent characteristics that make it most suitable for certain applications. For instance, the more resistant brands hold their shape well in detailed cane work, but they can be more difficult to condition. Some easy-to-condition clays become brittle after baking. Most brands feature a translucent color in their palette, but some brands are more translucent than others, making them more desirable when mixing tiny inclusions (such as glitter) into the clay or layering thin sheets or slices. Some clays change color after baking, making exact color matching more challenging.

• Sculpey is easy to condition and is also strong and flexible after baking. It has a tendency to become quite soft after repeated manipulation.

• Kato polyclay is both extremely strong and flexible when baked. It is easy to condition and its color does not change or darken regardless of how many times it’s baked. One of its best features is that it never feels sticky or too soft, even after being worked for long periods of time.

• Fimo Classic is more difficult to condition than Kato but is also great for caning. It holds detail well and is very strong after baking, but without much flexibility.

• Fimo Soft is easier to condition than the classic variety but it is not as strong and durable after baking.

• PREMO! Sculpey easy to work right out of the package. Comes in artist’s colors.

Health concerns

The safety of using polymer clay is the subject of controversy, specifically the long-term effects of exposure to the phthalate esters used as plasticizers to keep the clay soft and workable until curing.

The Arts & Crafts Materials Institute in Boston has tested the major brands of polymer clay and states that they comply with the ASTM D-4236 standard for safety of arts and crafts materials specified by the Consumer Product Safety Commission. A study conducted at Duke University Health System summarized the toxicology, metabolism and pharmacokinetics of the phthalate esters used in polymer clay formulations and evaluated contamination rates during laboratory simulations and use by professional artists. The study found actual transfer rates to skin, food, and mouth to be lower than expected and ingestion rates significantly below established Acceptable Daily Intake (ADI) for all phthalate esters tested. However, the Vermont Public Interest Research Group, Inc. has concluded that the use of polymer clays may lead to unsafe levels of exposure to phthalate plasticizers, especially in children, through accidental ingestion of residue on hands after use.

Restrictions on use of certain phthalates take effect in 2009 in the European Union and in the U.S. state of California. Polymer clays are being reformulated with less toxic plasticizers to meet the new standards.

It is known that incorrectly curing polymer clay by baking it at a temperature higher than that recommended by the manufacturer can produce fumes which may be harmful. However, correctly curing the clay, while it will produce an odor, will not produce harmful fumes.