Casting Glass Kiln Casting Slumping Fusing Techniques Molds Dams Guide

Working with glass in a kiln opens up a universe of creative possibilities, transforming solid sheets, chunks, or grains of glass into flowing, sculptural, or fused forms. Three core techniques dominate this fascinating craft: kiln casting, slumping, and fusing. While often used in conjunction, each process leverages heat and gravity in distinct ways, requiring specific approaches to molds, dams, and firing schedules. Understanding these fundamentals is key to unlocking the potential hidden within your kiln.

The Magic of Heat: Understanding Kiln-Formed Glass

At its heart, kiln forming is about controlling the viscosity of glass. As glass heats up, it softens, becoming increasingly fluid. Different temperature ranges correspond to different working properties. Lower temperatures might allow glass to gently bend or slump over a form, while much higher temperatures are needed to melt glass completely so it flows like thick honey into a detailed mold (casting) or to fully fuse separate pieces into one seamless whole. Central to success is the kiln itself. Modern electric kilns offer precise temperature control through programmable controllers. This allows artists to meticulously plan firing schedules – the sequence of heating ramps (rate of temperature increase), holds (soaking at a specific temperature), and cooling ramps (controlled temperature decrease). Proper cooling, especially the annealing phase, is critical to relieve internal stresses built up during heating and shaping, preventing the glass from cracking later. Another vital concept is the Coefficient of Expansion (COE). This measures how much glass expands when heated and contracts when cooled. For pieces involving multiple glass components (like fusing), using glasses with the same COE is usually essential. Mixing incompatible glasses will cause stress during cooling, inevitably leading to cracks. Most art glass is tested and labeled with its COE (e.g., 90 COE, 96 COE).

Kiln Casting: Filling the Void

Kiln casting involves melting glass within a refractory mold to create a solid glass object. It’s a way to achieve thick, sculptural forms often impossible with other methods. The process starts with creating a mold.

Mold Making for Casting

Molds must withstand high temperatures without degrading or sticking excessively to the glass. Common types include:

• Plaster/Silica Molds: Made from specific investment mixes designed for glass casting. Often used for lost-wax casting or creating detailed open-faced molds from a model. These are typically single-use.
• Sand Molds: Dampened casting sand can be packed around a model to create a simple, often textured, open-faced mold. Good for simpler shapes.
• Ceramic Fiber Molds: Rigid fiber boards or moldable fiber blankets can be shaped and hardened to form reusable molds, often for simpler geometric shapes or textures.

Before use, molds, especially plaster-based ones, must be thoroughly dried (candled) in the kiln at low temperatures to remove all moisture. They are then coated with a separator, often called kiln wash or primer, a refractory mixture that prevents the molten glass from bonding to the mold surface.

Filling and Firing

Molds can be filled with various forms of glass:

• Billets: Large, pre-cast blocks of glass.
• Cullet or Nuggets: Fist-sized chunks of glass.
• Frit: Crushed glass available in various grain sizes (powder, fine, medium, coarse).
• Sheet Glass: Cut pieces stacked in the mold reservoir.

The amount of glass needed must be calculated carefully, considering the volume of the mold cavity. Firing schedules for casting involve slow ramps up to high temperatures (often 1450-1550°F / 788-843°C or higher, depending on the glass and desired flow) with extended soaks to allow the glass to melt fully and flow into the details. The cooling cycle, especially the annealing range (typically around 900-1000°F / 482-538°C for common COEs), is critically important and often very slow, sometimes taking days for thick pieces, to prevent thermal shock and stress.

Slumping: Shaping with Gravity

Slumping uses heat to soften a piece of sheet glass just enough for gravity to pull it down and conform it to the shape of an underlying mold. It’s used to create plates, bowls, platters, curved panels, and textured surfaces.

Molds for Slumping

Slumping molds define the final shape. They can be concave (forming a bowl) or convex (forming over a hump). Popular materials include:

• Ceramic Molds: Commercially available or custom-made, durable for many firings.
• Stainless Steel Molds: Often used for simple curves or plate shapes.
• Fiber Molds: Rigid or soft fiber materials can be used, sometimes offering unique textures.

Like casting molds, slumping molds must be coated with kiln wash or a specialized primer to prevent sticking. Reapplication is needed periodically.

The Slumping Process

A piece of glass, often cut precisely to fit the mold dimensions or desired outcome, is cleaned thoroughly and placed on or in the prepared mold. The kiln is then heated to slumping temperatures, typically lower than fusing or casting temperatures (around 1150-1300°F / 621-704°C). The target temperature and soak time depend on the glass thickness, mold shape, and desired degree of slump (a gentle bend vs. conforming to every detail). Faster heating ramps are common compared to casting, but the cooling and annealing phases remain crucial.

Fusing: Joining Glass Together

Fusing involves heating two or more pieces of compatible glass until they melt and bond together permanently. This technique is used to create layered designs, intricate patterns (like mosaics), or simply to join components.

Coefficient of Expansion (COE) is King: Always ensure all glass pieces intended for fusing together share the same COE. Mixing COEs is a primary cause of cracked and failed projects. Check manufacturer labels carefully before combining different glasses in a fusing project.

Levels of Fusing

Different temperatures achieve different fusing effects:

• Tack Fuse (approx. 1350-1400°F / 732-760°C): Pieces stick together firmly, but retain much of their individual shape and edges. Creates a textured, dimensional surface.
• Contour Fuse (approx. 1400-1450°F / 760-788°C): Edges soften and round, pieces begin to merge more noticeably, but layers remain distinct.
• Full Fuse (approx. 1450-1500°F / 788-815°C): Glass becomes very fluid, pieces merge completely into a single, smooth, flat layer, typically around 6mm thick if starting with two standard 3mm sheets.

The Fusing Process

Glass must be impeccably clean. Pieces are cut, arranged on a kiln shelf (protected with kiln wash or fiber paper), and fired. Firing schedules vary significantly based on the desired fuse level, glass thickness, and complexity of the piece. Faster ramps might be used for simple tack fuses, while full fuses often benefit from slower ramps and specific holds to allow air bubbles to escape. Annealing remains critical after any fusing process.

Molds and Dams: Controlling the Flow

Beyond standard casting and slumping molds, dams play a crucial role in controlling molten glass flow on a flat kiln shelf. Dams are temporary barriers used to contain glass as it melts and spreads, creating specific shapes like thick slabs, geometric forms, or controlling the outcome of techniques like pot melts (where glass melts from a container suspended above the shelf).

Dam Materials and Construction

Dams are typically built from:

• Fiber Board: Rigid refractory boards cut to shape and joined with high-temperature wire or pins.
• Kiln Bricks: Soft insulating firebricks can be arranged to form barriers.
• Specialized Damming Systems: Some manufacturers offer reusable metal or ceramic damming components.

The inside surfaces of dams must be coated with kiln wash. Joints need to be sealed carefully, often with fiber paper strips or kiln cement, to prevent molten glass from leaking out. Dams allow artists to create thick, fused slabs that might later be slumped or coldworked.

Safety First, Always: Working with kilns and glass involves significant hazards. Ensure proper ventilation to remove fumes during firing. Always wear appropriate safety glasses when cutting or handling glass, and heat-resistant gloves when dealing with hot kilns or materials. Never open a kiln at high temperatures unless performing a specific hot-working technique with proper training and equipment.

Embarking on Your Kiln Glass Journey

Kiln casting, slumping, and fusing offer endless avenues for artistic expression. Starting with simple slumping or fusing projects is often recommended for beginners. Learning to program your kiln controller accurately and understanding the properties of your chosen glass are fundamental steps. Careful record-keeping of firing schedules and results will accelerate your learning curve. While the techniques have distinct requirements, they share the common elements of heat control, careful material preparation, and the critical annealing process. With patience, practice, and a respect for safety, the transformative power of the kiln awaits.