# The Additive Manufacturing of Glass: A Critical Review
## 3 A Focus on Semi-Solid Processing for Glass AM
### Postprocessing of AM Glass  
To date, comparatively little exploration of the postprocessing of additively manufactured glass has been reported in the open literature. A small number of papers have documented cutting and polishing objects produced by glass AM to further characterise the object [10; 22; 31; 67; 68]. While the polishing process was not the area of investigation for  
Figure 24: Various soda–line glass filaments manufactured using an MGP Vitri-GlassTM apparatus. Typical diameters (which are highly reproducible and customisable) range from 1 mm to 7 mm. Filament materials include the following: (**a**) opaque green Bullseye glass, (**b**) clear Bullseye glass, (**c**) yellow Kugler glass, (**d**) dual-colour filament in blue + clear glass, and (**e**) tri-colour filament in clear + orange + blue glass. these studies, it was still shown to yield some improvement in optical quality, reducing the visibility of the layer lines which distort the surface in a typical AM process. Kiln firing has also been attempted as a starting point for the postprocessing of layer lines. Many AM glass techniques involve the use of a heated chamber, annealing the produced object at the onset of printing and extending it after the process is completed [10; 88; 89]. Increasing the chamber temperature beyond the annealing range initiates a fusing and slumping process which reduces the visibility of the layer lines, at the cost of precise object geometry. An example of a 3D-printed and re-fired glass component via the use of a kiln is depicted in Figure 25. Investigation directly aimed at these methods of postprocessing is needed to characterise the complexities between layer line visibility, the resolution of object geometry, and ease of application. As an extension to kiln fusing and slumping, fibre optic applications also offer a specialised avenue for the postprocessing of glass AM objects. There have been multiple studies which demonstrate the successful drawing of fibre from a glass preform produced by AM [50; 65; 66; 67; 68; 82; 98; 99; 100]. In these studies, an AM process constructs glass into a complex preform geometry, which is subsequently heated to a suitable fusing temperature where it may be drawn into a fibre, free from layer lines and other artefacts of the AM process. In an examination of postprocessing through a more structural lens, hot working has the potential to further expand the extent of applications. An analogous concept is the welding of polymeric or metallic components. Such a procedure expands the possible size of final components or provides an opportunity to create a unique object geometry. The (seamless) joining of glass printed parts would have a particular impact on large-scale applications such as architecture/building. In the context of glass postprocessing, hot working can consist of re-firing 3D-printed glass with the use of a kiln, or through open-air glassblowing/forming techniques. The postprocessing of 3D-printed glass components utilising glassblowing techniques entails the manipulation of glass material at temperatures (and the associated viscosities) conducive to both sculpting and fusion. To date, demonstrations of this process have been seen in artistic exhibits or showcases in person. One such demonstration took place at AusGlass 2023, where six glass 3D-printed boats were amalgamated into a singular glassblown vessel, by glass artists [PERSON] and [PERSON]. Similarly, at the Glass Art Society conference (2023), glass artist [PERSON] revealed the integration of glass 3D prints to form a goblet, depicted in Figure 26. Across such demonstrations  
Figure 25: Re-firing 3D-printed glass (Bullseye glass) within a kiln. (**a**) Three-dimensionally printed form prior to firing. (**b**) Slumping cycle (700 \({}^{\circ}\)C held for 10 min) revealing lines from 3D printing still intact. (**c**) Extended firing cycle (held at high temperature until consolidation of mass). Printed lines have fused into one another, and the surface consists of a smooth texture.