# The Additive Manufacturing of Glass: A Critical Review
## 3 A Focus on Semi-Solid Processing for Glass AM
### High-Viscosity Glass Feedstock Methods
#### 3.3.1 Sustainability in Glass AM Through EAM and Example Models  
When compared to the range of possible glass AM techniques (Table 1), semi-solid processing emerges as exceptionally versatile regarding feedstock material demands (though it should be noted that the finest length scales may not be accessible to the EAM approach). Additionally, waste glass also stands out as an important material choice (where virgin material is not essential) due to sustainability benefits. In many instances, the utilisation of 100% waste glass is feasible for EAM, without necessitating the addition of any virgin material. Examples of 3D-printed designs fabricated from fully recycled consumer wine bottle glass waste are depicted in Figure 22.  
An example of an EAM-prepared glass model is shown in Figure 23. This is provided to highlight the breadth of geometries, contexts, and forms in which glass AM is being explored, including architectural models. Moreover, glass AM through semi-solid processing has some additional benefits in terms of volume production and scaling, through what are relatively low energy requirements to power semi-solid processing. This efficiency is primarily attributable to high-temperature processing confined only to the nozzle. Glass processing methods at a production scale typically rely on large furnaces that are continuously holding the entire glass material at ~1200-1600 \({}^{\circ}\)C over multiple years [8]. Regarding the high-viscosity glass feedstock methods (detailed in Section 3.3), in such cases, the majority of glass feedstock is at room temperature without an energy cost; any energy usage is specifically for the facilitation of heating and deposition precisely where required on a produced object. There  
Figure 21: (a) A 3D model ‘sliced’ using Simplify3D®, (b) the same model from (a) following 3D-printed production in 0.25 mm glass layers using an MGP instrument, and (c) a magnified image showing 0.1 mm glass layers, compared to a human hair (viewed under a digital microscope). is secondary energy usage from the heated chamber and movement system, but these are still less energy-intensive than maintaining a furnace of molten glass. In the context of the sustainability and recycling of glass, across all glass manufacturing techniques, devitrification is a concern when reusing glass. Devitrification, characterised by the transition of glass from its amorphous structure to a (partly or fully) crystalline structure, typically occurs due to the presence of impurities and exposure to specific temperature ranges conducive to nucleation and crystal growth [90]. In the case of the reuse of packaging glass, whilst it is known that such glass can be recycled many times (i.e., many dozens of times), there are often instances where, following a critical number of recycling cycles, glass may be more susceptible to devitrification. This phenomenon is, however, not well quantified in the field of glass AM, and most certainly is an avenue that merits future work. To date, issues of devitrification have not been critical concerns in the domain of semi-solid glass AM. This is possible because during AM (of semi-solid  
Figure 23: An architectural model printed with an EAM process out of recycled glass (specifically, recycled Gafter Glass). The model is a 1:20-scaled model of the Qaammat Pavilion, designed by [PERSON]. Toolpath generated by [PERSON]. Exhibited at Venice Glass Week 2023. Figure 22: Fashion pieces printed from recycled soda–time container glass (used wine bottles). Recycled filament manufactured using a commercial process from MGP, with 3D printing completed by Swarovski AG in 2021. Design credit to [PERSON] [97]. glass), any extruded glass experiences brief transit times through the nozzle--deliberately minimising exposure to temperature ranges associated with devitrification. This can possibly enable the processing of waste glass with fewer conventional challenges.