There are many plaster scrims (hoop cheese) to choose from when building your mold. It is essential to pick the right one to suit your needs. Here are some things to consider when selecting a material for casting plaster.
Use these guidelines to help create a quality mold:
• Scrim should be an open mesh fabric with no coating on the fibers so that nothing blocks or restricts airflow through it during burn out. The best choice of material will have holes no more significant than 1/8” in diameter and span between 1/32”-1/16” apart horizontally and vertically. If the mesh is too fine, it just won’t melt well enough duringout because the holes are too small
• A fabric that makes a strong “hoop cheese” is not necessarily a good material for a plaster mold because the scrim will adhere less to the model/pattern and more to itself, making pieces harder to pull apart. This means more seam lines in your casting, making it harder to prep and more obvious when you paint. Look for scrim with loose weave rather than tight stitch patterns on the individual fibers. There should not be any solid areas that hold into place while burning out, so heavier fabrics of this type would have their downside of being too thick or just proving very stubborn during burnout
• Avoid using burlap or other natural fiber fabrics since they tend to make a durable fabric that will not burn out well. You can try them, but you’ll be better off using something else
• If you use synthetic fabrics such as polyester or nylon, they will melt and breathe better than the natural fibers. Synthetics do tend to fade when in direct sunlight for long periods and if in an environment with excessive moisture (such as around pools and in marine environments), so keep that in mind when selecting your scrim material
• Avoid materials such as cotton duck canvas because these fabrics are hard to stretch over your mold without tearing. They also tend to shrink after burning out and can leave visible seams on your cast if used. Cotton duck is often used to create gap-filling resins such as Magic Sculpt and Milliput, if that’s what you’re working with, but not for making plaster molds
• A tar-like coating is often applied to fabrics used in mold making because it gives the material a “sticky” feel when stretching. This can help prevent the scrim from shifting during burnout and make it harder to remove from your finished mold after heating. If possible, don’t use this kind of scrim unless you plan on building your mold without seams. Once heated in a kiln, the tacky finish will melt away and make removing pieces apart more difficult
• Synthetic netting is preferred over natural fiber nets because they have a thinner mesh and will burn out far cleaner without any of the tar-like coatings that’s applied to some fabrics
• Avoid using plastic or bird netting because they can melt into an unusable gooey blob instead of burning off evenly, leaving seams on your casting. Plastic nets also have holes too big for proper airflow during burnout, making it harder for them to melt away.
These are essential things to keep in mind when shopping for scrim materials and help you make quality molds.