# The Additive Manufacturing of Glass: A Critical Review
### Glass as a Processable Material  
As elaborated in the synopsis of techniques which are being currently explored for glass AM (in the following section), the processes for glass AM utilise the unique thermal behaviour of glass in order to produce an object. This is not coincidental, as the thermal properties of glass have been utilised in glass forming for centuries [9]. The unique relation between temperature and viscosity allows glasses to be shaped in a viscous liquid state. This is depicted in Figure 2, which presents viscosity (for soda-time glass) as a function of temperature. The range of viscosity (given that the y-axis in Figure 2 is a logarithmic scale) for glass covers more than 12 orders of magnitude over a range of approximately 1000 \({}^{\circ}\)C.  
A gradual change in viscosity according to temperature allows a suitable working temperature to be chosen, balancing various factors such as the precision of material deposition (influenced by viscoelastic response, die swell, etc.) and the speed of printing (influenced by the pressure required for adequate extrusion, the rate of viscosity variation in material, etc.). A temperature is chosen at a suitable trade-off, such that the glass is'malleable' and easily deposited but will not deform under its own weight and lose complex details. While  
Figure 1: Examples of complex glass structures prepared by additive manufacturing of glass by material extrusion, both utilising translucent Gaffer Glass and opaque yellow-coloured Kugler glass feedstock. **(a)** Weave toolpath, designed by [PERSON]; **(b)** spiral ribbon vase, designed by Triple G Workshop. extremely variable between techniques, a temperature relating to the working point of glass (\(\sim 10^{3}\) Pa-s) is commonly chosen as a starting point, from which smaller qualitative adjustments are made to optimise material flow and stability after deposition [10]. For example, observations during an extrusion-based additive manufacturing process may identify that glass is not holding its shape after extrusion, failing to capture precise geometry. To remedy this, the temperature of extrusion may be slightly decreased, causing extruded glass to become more viscous and remain in position once deposited. The decrease in temperature must strike a balance, however, as continuing to increase glass viscosity will eventually introduce issues with the mechanism of extrusion. Figure 3 reveals a smaller range of temperatures, and viscosity as a function of temperature, for various common glasses. It can be seen from Figure 3 that there is minor variation in soda-time glasses (depending on colour, with colour arising from controlled minor/trace changes in glass chemistry), whilst so-called artist glass (such as Bullseye glass) has a lower viscosity across a range of temperatures, and borosilicate glass has a markedly higher viscosity across a range of temperatures. In contrast, quartz glass (SiO\({}_{2}\)) is a type of glass that does not contain additives, resulting in working temperatures that fall outside the ranges depicted in Figure 3. The results presented in Figures 2 and 3 may also be calculated using a convenient user tool developed by [PERSON] [11]. The tool has a user-interactive web site that draws from the SciGlass database, providing the viscosity and physical property of glasses as a function of their composition. The thermal properties of glass are leveraged in the direct AM of glass. A laser or heated nozzle is used to heat the glass feedstock to a suitably workable temperature, such that the material may be readily extruded or melted upon the build area. Deposited as a hot, viscous liquid, the glass may fuse with a previously deposited layer, necessary for the structural integrity of the finished object. As a fused deposit begins to cool, it also begins to undergo an increase in viscosity towards a solid form. This ensures structural integrity during AM, as layers can be stacked vertically without the earlier layers deforming due  
Figure 2: Viscosity of soda–time glass as a function of temperature.