Step 12Warnings about plastic
iImage Information

I personally used plastic, HOWEVER, the plastic does melt a bit... and as a result becomes a golden color... I don't think this seriously affects, or limits you producing more engravings, but, it does show that the plastic suffers a bit... and will be no where as permanent as glass. It'll last a good while I suspect (I'm preforming tests on it yet) but, the glass will be the preferred medium, as, this should not pose a problem. It may cloud though, and in that case will have to be wiped with a cloth occasionally to clear it for the next engraving.
I have an idea to make stencils that might be a little more work and a little more expense but I think the end result would be better. There is a product called Looking Glass. This is a spay "paint" that creates a mirror like finish on glass. By masking off your glass with a paper or vinyl piece in the pattern you wish to burn then spraying the glass with the Looking Glass paint you should end up with a mirror with voids that are in the pattern of the design you wish to engrave. Only experimentation will tell how well it would hold up.
I was just trying to add another idea to this good project.
Now, :) let me add something...
It could be possible to add 3D to the shadows (some less dark shadows) by cutting away some parts of the stencil after half burning the complete silouette...
So, the new cut parts would be half burned while the rest would look as mentioned in this interesting article.
For example, the front log of the first picture, would look less burned, so more close to the observer...
Hurray, I have added something! :)