3-D printing in glass
3-D design in glass uses glass powder like a pate-de-verre or kiln technique. The glass powder is spread out on a bed and then 3D printed using a binding material. The parts that will become the model are hardened by a binder and the rest remains as glass powder. A new layer of glass is added and the process repeats itself. The fragile model is then lifted out of the powder and fired in a kiln. The binder evaporates and the model fuses. 3D printing cannot replace the firing process, but does make the mold obsolete (2).
Glass powder fuses to solid glass by increasing its temperature. Depending on the firing temperature and duration, the adherence and the surface of the glass grains changes from sandy granulation to a smooth body and surface. In 3-D printing, since the process is different from blown glass and the glass mass is not agitated during the kiln glass process, the glass contains more air bubbles than blown glass influencing the transparency of the final glass object. The finer the frit, the better the detail but the more prevalently that the air bubbles are trapped. This scatters the transmitted light and turns the object opaque (3).

Transparency depends on the index matching between glass grain and solvent and the size of the glass grains.
Glass grains become invisible when the solvent they are suspended in has the same refractive index as the glass. However, with 3-D printing, the solution is squeezed out of the paste under pressure and jamming with bubbling occurs. Polysaccharides caramelize at temperatures between 110ºC and 180º. Above 250?C caramel decomposes into carbon mono- and di-oxide, hydrocarbons, alcohols, aldehydes, ketones and several furan derivatives which are volatile. When 40wt% of polysaccharides is added to the glass water mixture, the caramel is trapped inside the sample, leading to an incomplete decomposition and discoloration of the sample. Therefore, the polysaccharide content has to be reduced drastically to avoid this effect.
The size of the glass grains is another factor but scattering of transmitted light only occurs when the scatter is bigger than 20 µm or smaller than 400 nm, independent of the refractive index difference between scatterer and surrounding medium. Particle sizes between 38 µm and 75 µm result in transparent samples with sufficient detail.

3D printing in cold glass
Cold glass means that glass is used in a strengthening component. The weight percentage of the glass component is only between 5 and 20wt% and fixation does not rely on fusing of the glass particles. The toughness and elasticity of natural polymers change with the amount of added glass particles. A low weight percentage of glass reduces shrinkage but the mechanical characteristics of the sample after drying are governed by those of the polymer. The sample becomes tougher but more brittle with increasing glass content. Beyond 50wt% of glass there is a decline in elastic strength. The elasticity of the polymer makes it possible to apply the glass polymer film on elastic substrates. The best results achieved so far are with materials with are index matched to the glass particles.

Conclusion
At the moment, the use of glass in 3D printing is still not very common but it holds some promises for the future. While when used for simulating pate-de-verre techniques needing firing, newer cold applications may result in a more environmental-friendly and cost-saving application of the use of glass.

References

1. http://i.materialise.com/materials
2. http://www.shapeways.com/blog/archives/401-you-can-now-3d-print-in-glass-with-shapeways.html
3. 3D Printing of Transparent Glass. http://www.hpl.hp.com/techreports/2012/HPL-2012-198.html