# The Additive Manufacturing of Glass: A Critical Review
## 2 Glass AM Techniques
### Direct Methods
#### 2.1.2 Selective Laser Melting  
Selective laser melting (SLM) techniques direct an energy source across a bed of powdered feed material. An early version of this approach was attempted in art by [PERSON], whereby focused light from the sun was used to melt desert sand [41]. Later processes followed the more developed procedures for SLM, using a laser as the energy source and a glass powder feedstock. An object is created by scanning the energy source across the powder bed to fuse the feedstock in a desired pattern, as depicted in Figure 8. Vertical features are obtained by lowering the object in the bed and redistributing a thin layer of powder feedstock on top. Therefore, 3D features are built layer by layer, fusing the newly distributed feedstock upon the existing layers. The redistribution of the powder bed presents a unique advantage of SLM methods, as any unfused powder may support the model as it is built. Overhanging and bridging features may be generated for 'free', where other methods would need support structures for this geometry.  
Figure 8: Schematic of the typical SLM process [22] employed for glass to date.  
Present SLM methods demonstrate the ability to produce solid glass objects; however, these methods introduce multiple issues for large-scale printing. The processes explored to date rely heavily on feedstock materials of controlled size, shape, and composition. SLM methods have typically used high-grade glass powders with a spherical particle shape (for better flowability and redistribution) and size ranging from \(\sim\)1 to 500 \(\upmu\)m [22; 42; 43; 44]. At the present stage of research, methods are not being explored for a low-cost feedstock such as recycled cullet. There are likely to be issues at scale due to the sourcing of high-grade feedstock, or the processing of low-cost cullet to such a form suitable for an SLM method. Definitive research is needed in this area.  
Also, the powder bed results in the incomplete melting of the feed material and porosity in the molten glass. This causes undesirable traits in the finished object. Illustrated in Figure 9, this process creates a mostly fused core surrounded by a rough, opaque outer surface of unmelted glass powder [22; 42; 43; 45; 46].  
This outer layer, as depicted in Figure 9, has poor transparency, causing printed objects to lack the typical aesthetic qualities desired in a glass object. Based on research presented to date in the field, it appears that a postprocessing procedure such as mechanical or flame polishing is needed for SLM-produced objects with specific aesthetic or optical requirements (although such postprocessing methods are not yet widely explored). Such surface modification/polishing procedures ultimately limit the complexity of models, as they then require adequate access to all surfaces. The poor fusion in this process also raises the need to better understand the mechanical stability of glass objects prepared by SLM. Defects within the solid object create points for stress concentration, increasing the likelihood of fracture. The flexural testing of SLM-produced samples has shown strengths significantly lower than those of float glass of a similar material [42]. This presents issues with objects of significant scale, as the weight of the object itself may exceed the inherent strength of the object possible with this technique. This merits a brief comment that metal-glass hybrids, where the proportion of glass powder was no more than \(\sim\)30% in a glass-metal powder blend, have been successfully produced using SLM [47]. In the work, the resulting objects following fusion were metallic, with glass metallurgically fusing with the (majority) metallic feedstock.  
Figure 9: Photograph of a gyroid lattice glass object (20 \(\times\) 20 \(\times\) 20 mm\({}^{2}\)) produced with the SLM process [42].