Grinding & Polishing

As mentioned in the section on materials, the downfall of industrial cement is its softness - 2 on the Mohs scale as opposed to 6 for glass - which results in a surface that cannot be fine ground or polished. I attemped a fine grind of one of the cement castings using wet aluminum oxide powder, the typical practice for fine grinding a glass mirror, and the results were at first encouraging, yielding a smooth, even surface, but as the casting re-dried, the surface become uneven again. Also, based on its color, it appeared unable to dry back to the same low water content it reached after casting, and this residual water content would negatively affect the casting's structural properties. A second significant problem was the appearance of pits as I ground away the surface. I assume these pits formed as air escaped during the drying process.

Another option I looked into was adding a glazing to the surface, such as one used in pottery, and then firing the casting in a kiln. After researching this method at local pottery shops, I found that all were unwilling to put a plaster casting in their kilns, since they thought it would explode. Becuase the decomposition temparature of Ultracal is around $1450 \ensuremath{^{\circ}}C$, the casting could not be heated to a very high temperature anyway.

I also tried spraying on the surface of the casting a coating of Alvin LabMetal, a sandable/grindable aluminum-filled resin used for smoothing out welding seams. The hope was that when polished it would yield a reflective aluminum surface, possibly reflective enough to eliminate the need for a vacuum aluminum coating. This proved to be wrong and the LabMetal had the same flaw as the cement, a low hardness which made it difficult to fine grind or polish.

It might be worthwhile to look into USG's Metal Casting Plaster which has a lower expansion and lower permeability.⁵

Footnotes

... permeability.⁵

For more information see http://www.usg.com/Product_Index/_product_index.asp?vProdCat=95&vFamily=6&vGreenLink=95