The Air-Dry Cellulose Clays
Introduction: The Air-Dry Cellulose Clays top
In their natural state, cellulose polymers make up the physical structure of plants. We eat cellulose in fruits and vegetables and call it "fiber." We live in houses made of wood, the cellulose of trees. We make our clothes from cellulose fibers such as cotton and linen. Even the paper you are reading now is made of ground-up wood fibers (cellulose).
Paper is made by crushing (or pulping) wood (about 30 to 40% cellulose) and then making a slurry (or slush) of the wood fibers in water. Acids are added to break the wood fibers down to smaller size and chlorinated bleach is added to remove the "wood color." Other chemicals are added to neutralize the original chemicals. The water is squeezed out between metal rollers. A flat sheet of rolled pulp comes out like dough from a rolling pin. When the flat sheet dries, it is paper. The remaining chemicals in the paper are what cause it to turn yellow and deteriorate with age.
If you look at paper under a strong magnifying glass, you will see tiny fibers that look like fuzzy pick-up-sticks all stuck together with spaces and holes in-between.
In most commercial grades of paper, clay is added to fill the spaces between the fibers. This makes the surface smoother so that ink will bond well, and increases opacity so that printing on the other side won't show through. Brighteners and whiteners are added to improve contrast. A binder (glue) is used to hold it all together.
Some cellulose clays are made of paper, some are not.
If you grind up paper and add more glue, you get papier maché. Since homemade papier maché is usually made from old newspapers, it usually has weak, short fibers, and lots of leftover chemicals. The resulting papier maché has little strength and deteriorates rapidly.
The length and strength of the fibers, the kind, amount, and fineness of the filler materials, and the characteristics and amount of the binder (glue) all affect the working properties of the cellulose clay. Differences in these key factors account for the differences between competing brands of cellulose clays.
In general, dry powders that must be mixed with water before using will be described as papier machés and pre-mixed, ready-to-use products will be described as cellulose modeling compounds. The term cellulose clay includes both of these types of products.
Cellulose Content top
The cellulose used in a cellulose clay can come from wood or from cotton.
Wood pulp contains from 30 to 40% cellulose. Chemicals are used to reduce the fiber size and to remove the cellular debris from the pulp. Bleach is used to make the resulting fibers "white." The chemicals are neutralized with other chemicals because it is less expensive than washing them out.
Cotton fibers are 98% cellulose. Simpler chemicals are used to break the cotton fibers into smaller sizes and are washed out before the pulp is processed. The fibers are naturally transparent, so bleach is not necessary.
Generally the term "100% Rag" indicates that cotton rather than wood was the source of the cellulose. A smaller percentage (for example, 25% Rag) indicates how much cellulose came from cotton with the remainder from wood. The significance of using cotton rather than wood is that there are much less residual chemicals that might cause deterioration.
Another alternative is "acid free" or alkaline process cellulose. In this case, different chemicals are used on wood pulp so that the residual chemicals do not cause deterioration. While this has recently become important in the paper industry, there is no indication that it is being used yet in the cellulose clay industry. (The paper industry provides the wood pulp for most other industries that use it.)
Fillers serve two purposes. They add bulk to the cellulose fibers to make a workable clay, and they add color since the processed cellulose is actually transparent rather than white. The most common fillers are white kaolin clay, a hydrated silicon polymer used in porcelain, and white volcanic ash, a dry silicon polymer. Other clays are sometimes used with the addition of white pigments. Papier maché, made from actual paper, will have colored clays mixed with white pigment because this is how paper is made.
Clay filler is made mostly from two kinds of volcanic rock.
Real clays are tiny particles of volcanic rock mixed with water. The clay particles have been worn away by erosion. The particles are solid and have rounded edges from grinding against one another.
Volcanic ash is tiny flakes of volcanic rock that are spewed into the air during an eruption. Either the ash never makes it into rock, or it fuses together to form pumice (rock foam).
The ash is crushed to the right size. The particles are not solid and have sharp edges. (The particles may be lacy or corrugated.)
It is not clear at this time whether sharp or rounded edges make any difference in the performance of the final cellulose clay product. Both types are used in different brands of cellulose clay.
The fineness of the clay particles (how well they fit between the fibers), the color of the clay particles, and the amount of clay in proportion to cellulose fibers are the primary ways that the filler affects the final cellulose clay.
The most common binder, or glue, is starch (another polymer of sugar). Baking flour (used in home-made papier maché) is the most common form. The advantages are that it's inexpensive and it's traditional. The disadvantage is that it tends to seal the surface so that a thick piece may take a very long time to dry completely on the inside.
Plaster can be used as the binder if the final sculpture can be formed within a few minutes. Plaster "sets" by incorporating water molecules into it's crystalline structure with any extra water lost through evaporation from the surface. Thus, it avoids the problem of inside drying. On the other hand, you must work with it within the time limit for "setting." Thicker objects are built in layers or sections to get more working time.
Preservatives are necessary to prevent mold. Both cellulose and starch make excellent food for bacteria. Unfortunately, not all cellulose clays include preservatives. Generally, the pre-mixed cellulose modeling compounds include preservatives while the dry powder papier machés do not. (At least as far as we can tell from the labels.)
Water Content top
Water acts in two ways in a cellulose clay: it provides lubrication so that the fibers and fillers can be modeled and formed, and it keeps the binder unglued. (Except for plaster binder, in which case it activates it.)
With too much water, the clay won't hold its shape.
With too little water, the clay may become crumbly if an attempt is made to reshape it. In most cases, water can be added to make the clay more workable.
With plaster binder, you must work within the time limit. The clay cannot be rewet.
To minimize problems with mold or deterioration, use distilled or de-ionized water.
Drying Process top
The drying process is considerably different for "setting" binders such as plaster and "drying" binders such as starch.
Solid cellulose clay pieces with "drying" binders dry from the exposed surfaces.
Initially, water evaporates from the outside of the piece.
As the outside of the piece becomes dry, moisture attempts to redistribute itself evenly within the piece.
Because the evaporation is still occurring whenever water gets to the surface, this results in a dry outside, gradually increasing moisture, and a wet center. Eventually, the center may dry.
On some cellulose clays, the outside seals when it dries. This prevents water escaping from the center. On thick pieces, the center may never dry. (This should not be a problem in pieces less than 1/2-inch thick.)
Because cellulose clay does not shrink significantly during drying, there is no cracking of the surface.
"Setting" binders dry by incorporating water molecules into their crystalline structure and ejecting any extra water by evaporation.
Products with "setting" binders are always sold as dry powders. Adding water activates the binder and causes it to set after a working time.
Sealing a piece that is not thoroughly dry, or using it as an armature under polymer (oven-bake) clays will result in disaster.
Sealing a piece that is not dry will lock the remaining water inside the piece where it will continue to act as a lubricant, may continue to keep the binder unglued, and will promote the growth of mold. The piece will not have strength and may deteriorate internally.
Using a piece that is not thoroughly dry as an armature under polymer clay may cause the polymer clay piece to break in the oven. The water remaining in the cellulose clay turns to steam in the heat of the oven. The polymer clay prevents the steam from escaping — until the steam pressure is so great that it breaks the piece.
Papier Maché Brand Names top
Information Not yet available.
Cellulose Modeling Compound Brand Names top
Information not yet available.
Surface Characteristics top
The surface of Creative Paperclay is similar to a very fine water color paper. It is smooth and porous, and will absorb water with dye or pigment.
(See also: "What is Creative Paperclay" ).
The surface of Celluclay is less fine, but it is used by some doll artists (notably Van Craig, Merchandising Designer)
The surface characteristics of the other brands in this category have not been reported by dollmakers at this time (1/93).
The surface of cellulose clay is fragile. It scores easily. Before or after painting, the surface of the cellulose clay can be hardened with multiple, thin coats of acrylic gloss or matt varnish, acrylic gel medium, or acrylic gesso. See specific instructions for sealing the surface.
When to Use a Cellulose Clay top
The cellulose clays are very lightweight and have a paper-like surface. They are not as strong as the polymer clays, but since they don't weigh as much, they are less likely to be damaged by a fall or a swipe.
Use a cellulose clay when weight is a specific problem (such as a doll that will be shipped to many shows) or when the painting technique you want to use works better on a porous surface.
Use a cellulose clay armature under polymer (oven-bake) clays where light weight is necessary with the surface characteristics of the specific polymer clay brand. See special instructions for using cellulose clay armatures with polymer clays.
Paint can be kneaded into cellulose clays before shaping the piece. We do not have information at this time on the comparative merits of using water color, acrylic paint, or dry pigments. We suspect that oil paints would not be a good idea. The importer of Creative Paperclay indicates that any water-based paint can be kneaded in. If you experiment, let us know for the next edition of this book.
Painting (Porous Surface) top
(See a later heading for painting on a sealed surface.)
The surface of cellulose clay is similar to water color paper. The level of smoothness can be controlled by sanding, polishing with a wet brush (except when a "setting" binder is used), or applying a texture to the wet surface with a wet fabric.
The cellulose fibers will accept most water soluble fabric dyes or transparent colors as well as pigmented paints. Be careful not to make the cellulose clay so wet that the features move or melt.
With a brush, use standard water colors and water color painting techniques, but keep in mind that the surface you are working on is not flat and level. You may prefer to work with or without using gesso to prepare the surface. Practice before working on a final piece.
With an air brush, any water soluble dye or pigmented ink should work. Test solvent based dyes or inks before working on a final piece. You may find solvent based dyes or inks easier to work with because they do not wet the clay.
Sealing protects the surface of cellulose clays and prevents the cellulose clay from absorbing airborne bacteria and chemicals that may cause deterioration. In addition, sealing seems to considerably strengthen cellulose clays and make small parts less likely to fracture, mash, puncture, or score.
The surface of cellulose clays can be sealed with lacquer, acrylic varnish, or acrylic gel medium. All are available in any art supplies store.
Use acrylic varnish or acrylic gel medium if the surface has been painted with acrylic paints before sealing, or if it will be painted after sealing with acrylic paints. Also use acrylic varnish or acrylic gel medium if the piece will be used as an armature under polymer clay.
Use lacquer if the surface will be painted with artist's oil paints after sealing. Do not use lacquer for sealing armatures that will be used under polymer clays.
Make sure that the entire piece is completely dry before sealing. If moisture is left inside the piece, it will not be able to escape after sealing. Approximately 24 hours of drying is required for each 1/4-inch of thickness of the piece. In areas of extreme humidity, more time will be required. In dry areas, less time will be required.
Note: Cellulose clay with a "setting" binder "sets" within 30 to 45 minutes and is completely dry within 24 hours, pretty much regardless of thickness.
You can also dry cellulose clays in a slow oven. Be sure the temperature does not exceed 200 degrees F. (The manufacturer of Claycrete recommends a maximum of 150 degrees F.) If your oven thermometer is not accurate, you may ruin the piece. Higher temperatures will cause the water to turn to steam and may damage the piece.
While you can make several test pieces of appropriate size, and cut one open each day until the center is completely dry, it is far easier to use an armature of wadded aluminum foil and never make the cellulose clay thicker than 1/2-inch. As an alternate, build up the cellulose clay in layers and allow each layer to dry overnight.
Paint at least three thin coats of sealer on the piece. Allow each coat to dry completely before adding the next coat. Apply additional coats until the desired surface gloss or matt translucency is achieved.
Be sure to seal the base of the piece after the other sides are dry. Otherwise, contaminants may enter through the base.
Painting (Sealed Surface) top
If the surface has been sealed with an acrylic varnish or acrylic gel medium, use artist's acrylic paints. If the surface has been sealed with lacquer, use artists oil paints. Water-colors cannot be used on a sealed surface.
Patching and Repairing top
The tensile strength of cellulose clay after it has been sealed is about the same as polymer clay. Bob McKinley, NIADA reports that: "you can snap a finger off — takes a little effort, but you can. Five minute epoxy doesn't work on it. Aleene's Thick Designer Tacky Glue sure does though."
Cellulose clay can be built up in layers, but this will extend the drying time of each outer layer as the water is initially absorbed inwards as well as outwards. To prevent premature drying and poor adhesion at the interface between layers, use a brush to dampen the surface of the piece before applying the new layer. It will stick better, but take longer to dry.
Astry Campbell, NIADA works in layers and allows each layer to dry overnight before adding the next layer.
Working (Drilling, Sawing, Sanding, Carving) top
When the cellulose clay is dry, it can be drilled, sawn, sanded and carved. The material is quite soft, so take care not to sand away too much. If your working will generate a lot of cellulose dust, learn to sculpt better. In the mean time, wear a dust mask and goggles to prevent inhalation or eye irritation.
The cellulose clays contain the same ingredients as paper. They are generally non-toxic by contact or ingestion. They are not known to be sensitizers. They will burn as will any paper, and may cause intestinal blockage if eaten.
Paper dust, created by sanding cellulose clay, may contain wood dust, mineral dust, and mold dust or mold spores. It may cause respiratory irritation or allergy, skin irritation or allergy, or other problems. As a precaution, use good housekeeping measures, proper ventilation, and respiratory protection, if necessary.
Airborne cellulose dust may also be highly inflammable or explosive.
For additional information, see Artist Beware, the Hazards and Precautions in Working With Art and Craft Materials, by Michael McCann, PhD).
General Techniques top
As with porcelain clay, and air-drying clays in general, there are two major problems with cellulose clays: water content when you want it, and water content when you don't want it. Because cellulose clay doesn't shrink as it dries, the surface will not crack even though the outside dries faster than the inside.
Cellulose clay has a tendency to not stick to itself because of differences in moisture content between layers or pieces. Use a brush and water to moisten the two surfaces to be joined. They will bond properly and with no loss of strength.
You can layer the different brands of cellulose clays, but we do not recommend mixing them. Depending on the size and form of the piece, the following techniques are used.
For pieces up to one inch thick, work directly in Creative Paperclay for heads and use wire armature wrapped with white floral tape for limbs or fingers.
For pieces up to one-quarter life-size (18 inches), use a layer or two of Creative Paperclay over an aluminum foil skull, or over a skull-shape carved from a foamed plastic egg. If needed, use a foil and non-rusting wire armature for the figure. Caution: do not dry pieces with a foamed plastic armature in the oven.
For larger pieces, use a three layer method for economy:
Build a skull from a ball of crinkled aluminum foil or by carving a block of foamed plastic. Build a skeleton, or form, from artist's aluminum wire. Crinkle strands of inch-wide aluminum foil and wrap it spirally around the armature to provide more surface for the clay.
Build up layers of papier maché on the armature to within a half-inch of the final surface.
Sculpt the final surface with Creative Paperclay.
Cellulose clays are subject to deterioration by any of the bacteria, molds, or insects that eat cellulose or biological binders such as starch. The preservatives included in some formulas should prevent bacteria or mold problems initially, but the surface must be sealed to maintain protection and protect from insects. Cellulose clays that do not include preservatives may develop mold or a sour smell if left in a wet condition for more than a week. To help prevent mold growth or souring of mixed clays, use distilled or de-ionized water for mixing or softening the clay.
As a porous material, cellulose clay is also subject to chemical deterioration caused by airborne sulfur dioxide (which becomes sulfuric acid) and other industrial pollutants. Proper sealing of the surface will prevent penetration of airborne chemicals and prevent this problem.
Cellulose clays may also be subject to deterioration caused by ultra violet (UV) light. This should not be a problem unless dolls made of cellulose clay are placed in direct sunlight or in a window display for long periods of time. In this case, special UV screening sealers are available where photographic supplies are sold.
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