Yes and no.
A 3/4" thick 8" blank is more than thick enough. The 1:6 standard is outdated and I wouldn't follow it for anything larger than 6 inches. Generally anything around or thicker than 1:10 to 1:12 in ratio doesn't need a super complicated support either while grinding or in use in the scope, and is desirable due to the lower weight, cost, and cooldown time. I would think you could get a 3/4" thick 8" disk from many glass suppliers and if not ordering one is relatively cheap, probably less than $50. Even if that's not so, blanks show up on CN/Astromart and sometimes eBay often enough that I wouldn't contemplate anything like fusing glass together for an 8-incher.
That being said:
Mirrors in the 1:14 to 1:18 or 1:20 thickness ratio are generally the thinnest that can be made conventionally, and the greater the proportion of the thickness is eaten up by the depth of the curve of the mirror the more difficult it is to figure one or support it adequately. The old 16" x 1" plate glass portholes used by John Dobson are at the limit of what most mirror makers are comfortable with. I have a 0.8" thick 14.7" but it's quartz which is a little stiffer than regular glass.
Mirrors thinner than 1:20 (such as Mel Bartels' ~1:50 25" f/2.6 mirror) have been accomplished by amateurs by making them meniscus-shaped so that the curve is convex on the back to the same degree it is concave on the front side. This makes the mirror behave somewhat as though it's of whatever thickness the curve depth + glass thickness is, and helps ease the support requirements, though they are still pretty "floppy" and certainly not advisable for a beginner. The jury is kind of still out on whether or not these can become a more "mainstream" and practical approach for large mirrors as only a handful of (expert) ATMs have ever bothered with one and even Mel had a fair amount of difficulty - though now that techniques for working with them have been established, it might be easier.
It's possible to "sandwich" together glass slabs into a thicker mirror but this process requires a lot of knowledge of glass work. If you're going through the trouble of a kiln, etc. in the first place you probably would want to try a cellular design which is hollow, allowing air to flow through and reducing weight and glass use. These have been achieved to various degrees of success by companies such as Stabilite, Hextek, Fullum, Hubble Optics, and BVCTek the latter three of which are currently offering finished mirrors and blanks which work really great. The key to a good blank with long-term stability and no print-through problems from my understanding is actually fusing the glass on a molecular level as opposed to "gluing" it with some kind of adhesive, which is quite difficult to accomplish. Stabilite had a lot of QC issues, Fullum's TechnoFusion process is extremely expensive, Hubble's process seems to use adhesive at some level of the process and BVCTek's results in a "wavy" mirror back which may not be the most desirable.
Fusing together solid sheets of plate glass to make a thick blank can be accomplished somewhat more easily but isn't the most practical approach. One of the issues is that it's hard to fuse them absolutely perfectly and bubbles and other issues always result between layers, so the sagitta of the mirror cannot go below the top layer which can be a bit of a nuisance. That being said, I've seen a few successful examples. Chris Fuld's 41" Dob uses something like a dozen sheets of plate glass fused together, but I recall hearing that he actually had issues with the glass devitrifying on one side of the blank due to the amount of heating/cooling it had to go through. Another guy has been making a lot of (small) fused blanks as well as unsuccessfully attempting cellular blanks with a really basic kiln setup.
Edited by Augustus, 23 July 2021 - 09:52 AM.