Originally Posted by
Jesse Anderson
Glass does in fact have water in it i think.. Referencing a article i read on the internet... Because glass is a “natural” material (that is, its primary elements come from nature), one would think a CO2 laser would engrave glass pretty well. Ironically, it really does not engrave glass at all but it will mark it and with that contradiction, we will begin to understand how a CO2 laser beam interacts with glass.
In the most basic of terms, glass is made by heating silicon (sand). When heated hot enough, the silicon melts and becomes a thick, gooey fluid. This can then be poured into molds or blown by hand. As the molten glass cools, it remains transparent. To add strength or color to the glass, other elements are added. These are often metals such as lead, zinc, cobalt or even pure gold. In the case of lead, a considerable amount can be added to glass without affecting the transparency of the glass as you can see in 24% leaded crystal. 24 Karat gold on the other hand, produces a unique colored glass called cranberry.
Understanding this part of the process is important to laser users because, as all experienced laser users know, you cannot mark metal with a CO2 laser and that is just as true when the metal is molten (in glass) as it is when metal is in a sheet form.
So, thus far, we have learned that CO2 lasers can’t engrave glass, nor can they deal with the metal content of glass, yet we have all seen glass items marked by lasers. What gives?
This is really not as much a paradox as it might appear. Remember how glass is made? Silicon is heated to 2000 deg or more until it becomes molten. It is then removed from its oven and manipulated in some way. This introduces two things into the glass beyond its basic elements: air and moisture.
It’s the air and moisture that’s trapped within the glass that makes laser marking possible with a CO2 laser.
As a laser beam strikes the glass, it heats up the glass elements including the silicon and any metal content but neither of these elements will react to the relatively low heat and frequency of a CO2 laser. What does react is the air and moisture trapped between the elements of silica and metal. Both water and air expand when heated. Because the glass itself is relatively rigid, when molecules within the glass are heated until they expand, something has to give to allow for the expansion. This results in microscopic fractures in the glass, especially at the surface of the glass. It is this chipping or fracturing that we see as engraving.