Category: glass formulation

Glass Formulation Update

I brought the furnace up again in winter of 2016, made a bunch of material for casting (cullet, cane, shard, billet), and finished some additional testing on the batch and color compatibility.  I’ll post some notes about compatibility separately.  In short, its possible to melt batch glass with very little variation in compatibility from batch to batch, provided that the formula doesn’t deviate.

Compatibility is assessed qualitatively, with polarized light.  Typically, I’ll make a cookie (a disk) from two samples of cullet by melting them together in the glory hole, being careful not to swirl them.  After annealing, I look at their interface under polarized light for evidence of stress.  This method is quick, efficient, and reproducible.

I also further reduced the fining agent to 0.02%, and found a practical threshold of around 0.01%.  However, a formula without any fining agents may yet be realized.  The seeds are very small, and tend to disappear over the course a day, which is completely acceptable for my needs.  The formula was adjusted slightly to eliminate one of its more minor components, with no noticeable effect on compatibility or other practical characteristics (melt profile, working time, annealing profile, etc).

I’ve long had a hunch that a melt “ages” over time.  I did not find this to be the case however, at least over the course of a week.  Samples from the first day were highly compatible with samples from the last, and indistinguishable.  The cords at the very bottom of the pot likely arise from its slow and steady dissolution.  I’ve started scooping the last 8 – 10 kg of glass out before each new batch cycle, as it does affect slightly affect the new melt.

I love this glass, the formula, and the lack of variation.  I love the fact that I can make it inexpensively and consistently, and that I’m not reliant on glass companies to manufacture it for me.  I can hardly wait to kilncast a bunch of stuff with it over coming months.

Blowing Colors and Kilncast Compatibility

edit:  This post was originally written back in 2014, long before the environmental issues of Bullseye Glass and Uroboros Glass arose in the media.  Readers may find this post more relevant in light of current events.

A variety of Reichenbach and Gaffer colors (rods) were tested for compatibility and color durability with glass from my batch formulation in kilncast glass slabs.

Generally, most colors are compatible.  In particular, colors with lead bases are quite compatible.  Typically, the opaque reds, oranges, yellows, and some greens show significant incompatibility.  Test samples with compatibility issues readily develop fissures at the interface between glass and color, sometimes quickly and sometimes over a period of a couple of weeks.  A few colors exhibit “crazing” at the interface, but did not develop fissures of a more structural nature.  Some colors exhibit greater incompatibility with increasing thickness.  A superficial study color variation (from rod to rod) did not indicate a high degree of variation, however I plan to continue to spot check new rods of “compatible” colors until I get a better feel for the variation between different rods of the same color.

Many of the colors show a high degree of durability, meaning that they do not tend to change significantly due to the high temperatures and long firing times of kilncasting applications.  Some color groups do show significant change however.  Opaques tend to become transparent, but retain their color.  Copper reds tend to burn out to a liver color.  Some gold-based reds, oranges, and yellows tend to become pink in tone.  Overall, the changes are usually interesting and typically reproducible; I’ve developed a sample set to reference for future casting projects.


Glass Formulation

I have established a formulation for a dual-purpose glass which appears to be great for both blowing and kilnforming applications.  A dual purpose glass will facilitate the complete integration of hot-working techniques in art glass applications.  Spruce Pine 87 shows appreciable devitrification after only a single firing in kiln-casting applications.  Other commercially available batch glasses are marginally better than Spruce Pine 87, but tend to devit on the 2nd or 3rd firing.

The new formulation appears to be fully compatible with Spruce Pine 87 batch, and capable of multiple firings in kiln applications with minimal devitrification.  Of the Reichenbach and Gaffer colors tested so far, all were compatible in blown pieces, but some appear to be incompatible in castings.  The batch showed some stress (polarized light) with System 96 glass and with glass from Gaffer batch.  I hope to optimize the batch with the help of Gaffer, as they offer trident-seal testing.  They also trident-seal test their color batches as part of their production process.

The batch was produced with a common silica source and without decolorizer; its relatively colorless and suitable for thick applications.  The addition of 0.1% fining agent was sufficient to fine the glass; additional work will continue to further reduce this component.  The batch melts/cooks at typical temperatures (1230-1290 C or 2250 – 2350 C).  Materials are commonly available from pottery supply companies, and cost roughly $0.55 – $0.65 per pound.  A rotary cement mixer was used to mix the batch in 60 kg quantities; its poly drum has been fitted with a simple lid to keep to contain the dust.  The batch can be pelletized in the mixer with a sodium silicate solution.

Visit the blog in the technical-glass section for more detailed information.