Working with murrine, those captivating slices of patterned glass cane, pushes the boundaries of glass fusing. While basic fusing schedules might suffice for simple projects, achieving crisp, intricate details in complex murrine patterns demands a far more nuanced approach to kiln firing. Standard schedules often lead to blurred edges, lost details, or unwanted movement in the delicate internal designs. Mastering advanced fusing schedules is the key to unlocking the full potential of intricate murrine work, transforming patterned slices into stunning, coherent compositions.
The core challenge lies in the competing demands of the fusing process. We need enough heat and time for the glass components – the murrine slices and any base or surrounding glass – to bond securely and become a single, stable piece. However, the very heat required for fusion also makes the glass fluid, allowing the fine lines and colours within the murrine to shift, blend, and potentially disappear altogether. Intricate patterns, with their thin lines and close colour boundaries, are particularly susceptible to this heat-induced distortion.
Understanding Heatwork and Murrine Sensitivity
Heatwork isn’t just about reaching a target temperature; it’s the combined effect of temperature and time. Glass doesn’t instantly transform at a specific degree mark. Its viscosity changes gradually as it heats. Intricate murrine patterns require careful management of this viscosity curve. Too much time at too high a temperature, and the viscosity drops low enough for the delicate internal structures to flow and distort. Too little heatwork, and the pieces won’t fuse properly, leaving gaps or weak bonds.
Several factors inherent to intricate murrine complicate the process:
- Fine Detail: Thin lines and small colour areas have less thermal mass and are quicker to react to heat, making them prone to softening and blurring before thicker sections are fully fused.
- Differential Expansion: If the murrine cane wasn’t perfectly annealed or is made from slightly different glasses (even within the same COE range), internal stresses can cause distortion or cracking during heating.
- Trapped Air: Air pockets between murrine slices or between the slices and the base glass are common. If heated too quickly, this air expands and can distort the pattern or become permanently trapped as bubbles.
- Colour Stability: Some glass colours, particularly certain reds, oranges, and yellows, can strike (change colour) or react unpredictably at specific temperature ranges or with prolonged holds.
Designing Advanced Murrine Fusing Schedules
An effective schedule for intricate murrine focuses on control at every stage. It’s typically slower, involves more steps, and targets specific temperature ranges more precisely than a basic full fuse schedule.
Segment 1: Initial Heating Ramp
This phase brings the glass from room temperature up towards the process range. For murrine work, especially with many small pieces or thick assemblies, a slower ramp is essential to minimize thermal shock and allow heat to penetrate evenly. Cracking is a significant risk during this stage if heated too quickly.
- Rate: Often between 200-300°F (111-167°C) per hour. For very thick pieces or complex layouts, rates as low as 150°F (83°C) per hour might be necessary.
- Target: Ramp up to just below the bubble squeeze range, typically around 1000-1100°F (538-593°C).
Segment 2: Bubble Squeeze / Pre-Fuse Soak
This is a critical hold designed to allow trapped air to escape before the glass edges seal completely. As the glass softens slightly, the air can often find pathways out. Skipping or shortening this hold is a common cause of trapped bubbles in murrine work.
- Temperature: Hold between 1225°F and 1275°F (663-690°C). The exact temperature depends on the glass COE and the complexity of the setup.
- Duration: Hold times can range from 30 minutes to over an hour. Longer holds are needed for densely packed murrine or thicker projects. Observe carefully during initial tests if possible.
Segment 3: Ramp to Process Temperature
A moderately paced ramp takes the glass from the bubble squeeze temperature up to the top fusing temperature. Still slower than a generic schedule, but faster than the initial ramp.
- Rate: Typically 300-500°F (167-278°C) per hour.
Segment 4: Process Soak (The Critical Stage)
This is where the fusing happens and where detail preservation is paramount. Unlike a standard full fuse aiming for 1480-1500°F (804-816°C), intricate murrine work often requires lower top temperatures and potentially shorter hold times to minimize pattern distortion.
- Temperature Target – The Balancing Act:
- Lower End (Tack/Contour Fuse): 1350-1400°F (732-760°C). This range minimizes glass movement, preserving sharp detail. However, it may result in a less smooth surface and slight gaps between elements, requiring potential coldworking or a subsequent slump firing.
- Mid-Range (Controlled Fuse): 1410-1450°F (766-788°C). This is often the sweet spot for intricate murrine. It provides sufficient heat for good bonding and some smoothing, but with careful timing, limits excessive flow that distorts patterns. Soak times here are critical – often shorter than a full fuse.
- Higher End (Calculated Risk): 1460°F+ (793°C+). Approaching standard full fuse temperatures increases the risk of detail loss significantly. Only use this if a very flat surface is essential and the murrine design is robust enough to withstand it, or if soak times are kept extremely short (flash fusing).
- Soak Duration: This is highly variable and depends heavily on the chosen temperature, the thickness of the glass, and the intricacy of the murrine. It could range from just 5-10 minutes at higher temperatures to 20-30 minutes at mid-range temperatures. Testing is absolutely essential. Start with shorter soaks and increase cautiously based on results.
Important Kiln Variability: Kiln controllers and thermocouples can have slight inaccuracies. The temperatures and times provided are starting points. Always conduct test firings with your specific kiln, glass, and murrine type before committing a large or complex piece. Keep meticulous firing logs.
Segment 5: Rapid Cooling to Annealing Range
Once the process soak is complete, you want to cool the glass relatively quickly to stop the fusing action and “freeze” the design in place. This rapid cooling also quickly moves the glass through the devitrification range (roughly 1300°F down to 1000°F or 704°C down to 538°C), where cloudy crystals can form on the surface.
- Rate: As fast as possible (AFAP) or set a rapid rate like 9999 on the controller. Kiln size and insulation determine the actual cooling speed.
- Target: Bring the temperature down swiftly to the annealing soak temperature.
Segment 6: Annealing Soak
This is non-negotiable for structural integrity. Glass that appears fused can shatter days or weeks later if internal stresses aren’t properly relieved through annealing. The temperature and duration depend primarily on the glass COE and the thickest part of the fused piece.
- Temperature: Typically around 900°F (482°C) for COE 96 glass, or 950-960°F (510-516°C) for COE 90 glass. Consult manufacturer recommendations.
- Duration: Varies significantly with thickness. A quarter-inch piece might need 1-2 hours, while a half-inch piece could require 4 hours or more. Err on the side of longer soaks for complex murrine pieces.
Segment 7: Cooling to Room Temperature
The final cooling phase must be slow and controlled to prevent thermal shock, especially below the strain point (around 750-800°F or 400-427°C).
- Rate 1 (Annealing to Strain Point): 80-150°F (44-83°C) per hour. Slower for thicker pieces.
- Rate 2 (Strain Point to Room Temp): 150-300°F (83-167°C) per hour. Once below about 300°F (150°C), the kiln can often be allowed to cool naturally or turned off, but avoid opening it until it’s close to room temperature.
Advanced Considerations and Testing
Beyond the core schedule, consider:
- Damming: Using ceramic or fiber dams can help contain the glass and prevent intricate patterns laid near the edge from flowing outwards.
- Pre-Fusing Components: Sometimes, creating a sheet of fused murrine slices first, then firing that sheet onto a base glass in a second firing using a lower temperature schedule, offers more control.
- Coldworking Plan: If you intend to grind and polish the piece extensively, you might opt for a slightly lower process temperature, accepting a less smooth surface initially, knowing you’ll refine it later.
The Power of Testing
No article can provide a universally perfect schedule. Every kiln fires slightly differently. Different batches of glass, even with the same COE, can behave subtly differently. The specific design of your murrine – line thickness, colour density, complexity – dramatically impacts how it reacts to heatwork.
Run small tests first! Use scrap murrine slices similar to those in your main project. Test different process soak temperatures and times. Document everything: schedule used, glass types, layout, visual results (detail clarity, fusion level, bubbles, distortion). This iterative process of testing and refining is the only way to truly dial in the perfect schedule for your specific materials, equipment, and artistic vision when working with intricate murrine.
Creating sharp, detailed art with intricate murrine is a rewarding challenge. It requires patience, observation, and a willingness to move beyond basic firing protocols. By understanding the unique demands of these patterned glass elements and meticulously controlling the heatwork through advanced, tailored fusing schedules, artists can preserve the delicate beauty captured within each slice, transforming them into stunning fused glass masterpieces.
