Just wanted to throw this out as an FYI.
Before I got my granite plate, I used glass for lapping all my hand tools.
I found a very inexpensive way to get tempered glass.
I went down to my local glass company and explained what I was looking for (tempered glass for sharpening my tools) and they gave me a piece for free. When they temper glass they usually do a couple more in case one breaks during the process.
The glass I got for free was for the windows in a fireplace. It measured 4 1/2 x 12 inches. Perfect for me when I was doing plane blades and chisels.
This is also a good way to get discounts on glass for projects, especially if your project can work around the size of the glass a little. Since they sometimes have these extra pieces from larger projects, they offer them at reduced prices. A lot cheaper than special ordering tempered glass.
Just thought I'd share! All have a great day!