Section 2.13.1.
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Glassmaking on the Moon

Geoffrey A. Landis

Glass is a very useful material for the Moon. However, it is considered such a low-technology, bulk thermal process that it is hard to realize that it may be difficult on the moon. It is desirable to design a process that can use un-beneficiated regolith.

Glass is an amorphous mixture of silicon dioxide with typically alkaline and alkaline-earth oxides. The silicon dioxide forms the structural network of the glass; the other oxides modify the properties to the desirable form. On earth, the most commonly produced glass is soda-lime glass. "Soda" indicates sodium, in the form of Na2O; lime indicated calcium oxide, in the form CaO. The typical proportions are SiO2 75%, Na2O 15% and CaO 10%, plus 2-3 percent other materials. Soda lime glass is produced in large amounts on Earth because its low melt temperature makes it easy to work with. Unfortunately, sodium is not a common material on the moon, comprising 0.1% to 1% of the bulk lunar regolith. A second disadvantage of soda-lime glass is that it has a very high thermal expansion coefficient, making it vulnerable to the temperature excursion of the lunar day-night cycle.

An alternative glass is pure fused silica, which consists of silicon dioxide plus trace components. While silicon is abundant on the moon, the melt temperature of fused silica makes it extremely difficult to work with. Typically laboratory silica is mixed with other components to lower the working temperature.

A useful laboratory glass is borosilicate glass, such as pyrex, where boron oxide B2O3 is the primary additive, however, boron is also not a commonly available material on the moon (1-50 parts per million).

For this reason, I have selected aluminosilicate glass as the baseline for lunar glass. A typical aluminosilicate glass formula is SiO2 57%, Al2O3 20%, MgO 12%, CaO 5%, B2O3 4%, Na2O 1%, trace oxides 1%.

Of these constituents, silicon, aluminum, magnesium and calcium are common on the moon (in fact, calcium-magnesium aluminosilicate is virtually the definition of the anorthositic material that is the primary constituant of the moon, with some iron substituting for magnesium). I will assume here that the boron and sodium oxides can be left out. This will have the unfortunate effect of raising the softening temperature of the glass, but will allow it to be made from lunar material.

Aluminosilicate glasses have an intermediate melt temperature, typically ca 1130 °C, which is well above the 860 °C melt temperature of soda-lime glass, but well below the almost unworkable 1710 °C of fused silica. In terms of working temperature, it has an annealing temperature of typically 715 °C, and softening temperature 915 °C (although note that the boron-free formulation discussed here will probably have a slightly higher temperature). It also has a low thermal expansion coefficient, comparable to borosilicates.

The second difficulty, after choosing a glass composition, is that for most uses our glass will have to be transparent. This turns out to be a significant difficulty. Glass darkening is typically due to transition metal oxides, in particular, iron oxide (FeO). FeO is a significant component of lunar rock, ranging from roughly 3% to 23% of the rock weight fraction. This is why lunar rock is black. Producing a transparent glass will require reducing the iron content to essentially zero. On Earth, this is done by selecting iron-free source materials; however, this is not likely to be an option on the moon, and removal of the iron will probably have to be done by chemical or physico-chemical means. Another step used on Earth is to add manganese dioxide to neutralize some of the green color of the iron; however, on the moon, manganese is a low abundance mineral (<0.25%), and is found typically in exactly the same rocks that contain Fe, with a Fe:Mn ratio of ~80:1. Thus, this is not a good option.

In principle, in the procedure outlined here, most (if not all) of the iron is segregated in the form of an iron/aluminum/titanium mixture by potassium reduction. Of this, only the aluminum component is needed as a raw material for the glassmaking. Thus, the question of removal of the iron is equivalent to the problem of purifying the aluminum, discussed in the aluminum section.

Thus, there seems to be no real barrier to production of glass from lunar materials. Once the raw oxides have been refined, the glassmaking process requires melting the component materials, producing the required form (plate, cast ingots, etc), and, for stress-free glass, holding the glass at the annealing temperature for an extended period to allow internal thermal stress to relax. Conventional glass plate produced by the "float glass" method is done by spreading the melt on a bed of liquid tin; this may not be the optimum process for lunar production. The melting temperature for the aluminosilicate glass, on the order of 1130 °C, is significantly higher than the temperatures required in any other parts of the solar cell production process. This is an incentive for refining of boron, sodium, and lithium oxides despite the relatively low mass fraction of these materials in lunar soil, to lower the glass melting temperature.

An alternative approach to glassmaking, discussed by Mackenzie and Claridge [1], is to form glass out of anorthite purified from the lunar plagioclase. They note that anorthite (calcium aluminosilicate) is naturally low in iron, and hence can be used as a material to form a transparent glass. Their approach requires purification of the feedstock to separate out essentially pure anorthite, then melting this to make glass. The melting point of anorthite, approximately 1550 °C, is relatively high, making it difficult to work with. They suggest addition of calcium oxide, to form a composition of roughly 46% CaO,42% SiO2, 11% Al2O3, and 1% trace, to reduce the melting point to 1350 °C, and sketch a design for an electric furnace for processing the glass into both sheet and fiber. Even this modified composition results in a very high melt temperature of the glass, though, which will make it difficult to work. Nevertheless, if beneficiation of the input material to purify anorthosite proves to be easy, this may be a good approach.


1. Mackenzie, J. D., and R. Claridge, R., "Glass and Ceramics from Lunar Materials," Space Manufacturing Facilities 3, AIAA, NY, pp. 135-140, (1979).


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