This article was inspired by a visit to E. Dehillerin, a wonderful kitchen shop in Paris. While browsing the shelves I came across a large collection of pastry frames, stainless steel bands used by chefs to form baked items. I had seen similar rings sold to replace dams used to kiln form thick pieces of glass, but these kitchen items were significantly less expensive, quite sturdy, and came in a wide variety of sizes shapes, including squares, rectangles, circles, hexagons, tear drops and other shapes a creative pastry chef might want. I purchased quite a few of these forms and have since used them successfully to form thick designs, pot melts, and pattern bars. At the recent NCAGG meeting, this and other forming materials came up in the discussions, and I agreed to write an article summarizing what I know about the subject. I would appreciate hearing from other members about techniques not covered here.
A large variety of materials for forming glass are available at local glass shops, through online glass sources, and through other suppliers. This article covers some of the common materials and techniques for using them in slumping, damming, or doing drop forms.
Clay, Ceramic Molds, and Refractory Materials
High quality clay molds in a wide variety of forms can be acquired from most glass dealers and online sources such as Slumpy’s and Future Forms. Their porous surface easily takes and retains kiln wash. Materials used in these molds are formulated for durability and resistance to cracking under repeated firing cycles. Prefired bisqueware sold by pottery companies may also be used to slump glass, but may not be quite as durable.
Many fusers purchase odd pieces of pottery at home goods stores such as Crate & Barrel or Target and use them as slumping modes. Some of them have unusual textures that can provide impressive effects on fused glass. Large pieces of pottery or ones with complex forms should have holes drilled in strategic places to ensure air is not trapped between the pottery and glass.
It is tough to get kiln wash to properly stick to glazed pottery. Sand blasting or other forms of roughing the surface can improve the adhesion of kiln wash. Glazed pottery should be heated to a temperature of 400 °F in a kiln prior to coating with kiln wash so that the water quickly evaporates and the wash does not run. Wash can be applied by brush, garden sprayer, or air brush. Similar techniques work for other materials such as stainless steel molds. Some fusers use a Boron Nitride spray for slick materials, but this may not work well if there are complex curves in the piece, e.g., molds use for glass weaving.
Another common technique is the use of rectangular bars of heat resistant clay materials to prevent the spread of glass that typically occurs whenever multiple layers are fused. Damz bars, commonly sold in most glass shops are an example. Some fusers save money by cutting up used or broken kiln shelves into strips with a tile saw. These materials either need to be coated with kiln wash or lined with fiber paper to ensure release of fused glass.
Fiber Paper, Board, and Other Mineral Materials
Fiber paper and fiber board are sold in a wide variety of thicknesses ranging from Bullseye ThinFire to several inches thick. Their advantage is that they can be formed into many shapes. To be used to form glass they need to be treated with a hardener or supported by an infrastructure that will be rigid under kiln heat. Fiber paper and board are also commonly used to create channels or separations in fused items such as jewelry and vases. Multiple layers of fire paper can be joined together by glue or stapling to vary thickness. Fiber board can also be carved to create custom shapes for molds or drops.
Fiber hardener is sold as a liquid that can be used to soak fiber materials or as a pre-soaked fiber. When fired to kiln temperature the fiber material becomes rigid. One of my favorite materials for supporting fiber paper is a metal mesh available from Slumpy’s that can be molded by hand, but remains rigid at kiln temperatures. When supported by kiln bricks and covered by 1/8” fiber paper, large pieces can be slumped in unique shapes.
Use of fiber paper for damming hot glass is a bit of an art. On first thought it is tempting use paper up to the top of where the glass is piled. However, this results in “jaggies,” sharp threads of glass that stick to the top of the fiber paper. It’s typically acceptable to cut the fiber paper to 1/8” lower than the top of the glass and let the surface tension of the glass hold the top 1/8” in and take care of creating a smooth top.
Lava Cloth, a fiberglass material, imparts interesting textures to glass since it maintains its texture under kiln heat and does not stick to glass. Lava cloth seems to work for most colors if fired no higher than tack fuse temperatures and anchored properly to minimize shrinkage. Some fusers have had problems with jagged texture and curling, typically at full fuse temperatures.
Fiber cord in a variety of thicknesses is also available and can be used to create unusual shapes or vary the contours of a clay or stainless steel mold.
(Cautions: Make sure that you know the materials and safety issues associated with potentially cancercausing fiber. Whenever cutting any of this stuff, wear a respirator rated for fiber, vacuum up the leaving with a HEPA vacuum, and make sure the shop is properly ventilated.)
Plaster and Casting Compounds
Plaster/silica flour mixes and various molding compounds such as Hydroperm can be used for slumping. Procedures are similar to casting, and beyond the scope of this article. Disadvantages are the effort required to cure the mold and the short mold life, typically only a few kiln cycles.
Plain dry plaster of Paris is a surprisingly versatile compound for forming glass. Impressions made in compacted dry plaster can be transferred to sheet glass with high accuracy. The principal drawback is the amount of dust in the kiln. I use a large stainless steel baking pan to hold plaster when using it dry.
A casting box made of vermiculite board is another possible container. (Cautions: Silica flour contained in most molding compounds is considered to be cancer-causing. While mixing use a respirator and make sure the shop is ventilated.)
Stainless Steel is the preferred material for metal molds, although some other forms or iron may be usable. The principal advantage of stainless steel is that it flakes minimally. Other forms of iron shed flakes that can ruin your work, so the glass must be protected by a layer of fiber paper, and you will need to vacuum your kiln to clean metal flakes. Even stainless steel does some flaking that may require a wire brushing between uses.
As it cools, stainless steel contracts more than glass, so it is the preferred material for draping. Large slumps such as sinks can be done in woks and other types of metal pots or bowls that are available in home goods stores. If you are slumping into a large steel mold, be sure to drill some air escape holes.
If you have access to a metal brake, a device for bending sheet metal, you can form stainless steel sheeting (obtainable at hobby stores and sheet metal suppliers) into useful molds. Metal rods, flat bars, and corner pieces obtainable at hardware stores are also useful for damming if they are lined with fiber paper or heavily coated with kiln wash.
The same issues with kiln wash apply to stainless steel as to glazed pottery. If you don’t have access to a sand blaster for roughing the surface, it does help to prefire the stainless steel to 1400°F, or to work the steel over with sand paper.
(Cautions: Under no circumstances use galvanized or zinc coated metals. Zinc Oxide fumes that form at kiln temperatures are toxic. Aluminum melts at a relatively low temperature, 1220°F, and is not usable.)
Surprisingly, glass can be used to form glass. Borosilicate glass doesn’t soften until about 1500°F, so it is a useful material for glass forming. A wide variety of Pyrex rods, tubes, and flat bars are available from lab supply houses. I have not used this material myself, but Nikki O’Neill shared this technique with us at a NCAGG meeting, and I am looking forward to trying it.
Suppliers for materials mentioned above
Weisser Glass Studio (local source for supplies)
Vitrum Studio (local source for supplies)
Slumpy's (specialist in molds)
Future Forms Molds (specialist in molds, Future Forms)
Clay King (bisqueware)
Kerekes (cake, pastry frames)
Hot Glass Color & Supply (borosilicate in a wide variety of shapes)