Wednesday, June 7, 2017

Crash Course Casting

Cast glass is fascinating. It’s no wonder the sight of a meticulously executed piece sends your mind racing. You find yourself pulling up to admire and study it; to marvel at the bulky, sculptural character. You become captivated by the suspended design and colorful shards floating deep in space. You’re tempted to slide your finger tips lightly over the silky, smooth surface or to touch the magnified, grainy pattern embedded underneath. You guess how much it weighs; think about how it was made and calculate how long it took to fire.  

Before long the complex technicalities sink in and become overwhelming, quickly building a road block that brings your run-away inspiration to a screeching halt. But the ride doesn’t have to stop there!  

 Amazing glass castings come in all sizes; by starting small you maximize your success rate.  It enables you to work in multiples and test-drive several different types of techniques all at the same time. Plus the variety of projects will ramp up the production value and educational worth of the load making the commitment to a prolonged firing more agreeable and satisfying. Inevitably there will be surprises; some projects will come out far better than expected and others will lack luster. Good or bad though, there is something to be learned from every project whether the outcome is considered a success or not.

One of the things I like about making cast pieces is that the approach is different from straightforward glass fusing.  The focus of basic fusing is the project and how efficiently it can be made. Casting is driven by process.  It’s the sequence of multiple, singular steps carefully carried out over a period of time, seamlessly linked together that craft a engaging piece of art.                

Casting begins with a mold; a form that can hold-up to the high heat inside the kiln while containing glass in a desired shape. For single use molds, to make one-of-a-kind pieces I recycle damaged ½ inch thick fiber board kiln shelf material. I determine how tall I want the fired piece to be. I add 25% to that measurement if I intend to use small material like course frit or mosaic sized glass shards to fill the mold or as much as 40% larger if I plan to use big chunks. 

Cut single thickness, clear fusible glass to line the inside of the fiber board mold. The lining improves the smoothness of the fused glass, minimizing the amount of polishing that might be needed.  

Assemble the glass pieces on a primed kiln shelf in your kiln. Build a dam with the fiber board in a pin-wheel fashion around the glass.  Making sure the fit is tight to avoid any glass seepage during firing. 

Before filling the mold there are some things to consider. A mold filled with powder size clear frit will have a minimum amount of shrinkage. The finished piece will have tiny champagne like bubbles throughout, lack clarity and look milky. Fill the mold with fine clear frit and the number of bubbles will be reduced and the clarity somewhat improved.  Load up the mold with medium or course clear frit and the bubbles will be fewer still, but larger and the clarity will be greatly improved. The shrinkage will be considerable. You can compensate for this by over filling the mold, mounding the glass higher than the side walls in the center.  Casting rocks are another option. They are random sized chunks of practically bubble free glass formulated for casting. Using them can reduce the number of bubbles increasing optical clarity.  It’s my common practice to mix mold fillers, catering the materials to the individual project and desired outcome.  

The color used to fill the mold is yet another consideration. The beauty of cast glass is the transparency; the ability to see through the art. Medium colored shades of glass when piled up increase in density and result in unusually dark pieces. My advice would be to experiment with color and use it in moderation. Try a combination of applications like layering it with clear, choose pale shades over dark and use transparent glass as opposed to opal.  

The Evergreen Sushi and Rip Tide Sushi cast blocks are delightfully deceiving. The intricate embossed patters on the bottom in combination with the built in riser set these pieces apart from the others.  They look complicated, but in reality are quite easy to make. And, I might add I have had the same dynamic results making them in smaller sizes like 6” x 6” and even as small as 4” x 4”. Don’t hesitate to try this technique in any size; you’ll be thrilled with the end result.

These castings are built on a piece of fiber board with two vertical slots, for the feet cut all the way through. The board is then primed with kiln wash. The embossed design is drawn in pencil on 1/8” thick fiber paper and cut out a hobby knife. The cut fiber paper design is made firm with mold hardener and primed. The slots in the board are filled with glass. A single piece of single strength fusible glass is cut the size of the fiber paper and laid on top. The glass holds the loose embossed design pieces in place while the mold is filled.  

Cut primed fiber board into 2” wide strips. Cut the strips to fit around the clear glass base including the fiber paper design. Surround the glass and fiber paper with the fiber board to make the casting mold. Fill with casting rocks trying to keep the material from touching the side walls of the fiber board mold. If the glass can melt in the center then flow outward to the side walls it reduces the number of imperfections, sharp spikes along the project edges. This minimizes the amount of cold work needed. 

                Cold working is the process of grinding and polishing glass to make the surfaces flat, smooth and shinny.  Cast glass often needs some cold work touch up.  If you are not familiar with the equipment a local hot shop will be able to do the polishing for a fee.

Casting Firing Guide – Approximate time 42 hours

Segment              Rate Per Hour    Temp/°F               Hold/Minutes

1                            80                          300                       30

2                            80                          1000                     90

3                            225                       1450                     90

4                            9999                     960                       360

5                            50                          800                       60

6                            100                       600                       60

7                            100                       100                       0

8                            60                          60                        0

 The trick to successfully firing cast glass is patience.  It’s not a bad word!  Here again I pay tribute to the benefit of nesting multiple, small pieces in a single firing.  Greater value usually equals greater self control; usually.  


Rev up your engine, give casting a spin. On your mark! Get set. GO!  

Happy fusing,

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Upscale Fusing Webinar
July 25, 2017

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September 7, 2017

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October 26, 2017

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