I have used a couple of De Vere 10x8 mural enlargers for both colour (mainly) and B&W work. The biggest single prints we did were 72 colour wide paper by 18 long, this was a Kodak product.
The biggest B&W murals we were able to do were 48 (maybe 50) wide by 18 long, this was on Ilford paper. The major differences between the two were only that one was B&W the other colour plus the actual roll width difference.
This was RA4 for colour (prior to that EP2) and whatever Ilford B&W process was the go at the time. The time was the eighties through to the nineties.
As for reciprocity on murals, well within reason it doesnt matter, also it isnt that much, from actual experience here. Yep, it does happen, but to be honest, if you are doing a mural print youll do a test strip print, develop it, throw it on the floor under very good light, or walk outside and place it on the footpath under sunlight and check it out.
Many a time with maximum enlargements, one could be doing 15 to 20 minute exposures. This enlarger
http://www.deveresecondhanddarkroom.com/17-de-vere-508h-photographic-enlarger.html in a different model, or maybe this one, has a panel hiding an array of lights, one simply opened up the panel and sat right alongside the enlarger and read a book with large type for the duration. All of the darkroom workers doing murals did it that way, read large print books that is.
Basically with mural work you need to slow down, take your time and get used to the dark. The best mural darkrooms ran De Vere enlargers, I dont really think anything else I saw came close, doesnt mean anything else was no good, but the De Vere I believe was the top of the tree.
These enlargers from memory, ran a 2,000W array of globes, that is, they have 8x 250W enlarger globes in the head. It gets warm, the darkroom gets warm, great in winter, not so flash in summer. All of the vertical mural enlargers Ive used always used glass, but you could also mount without glass, which is what we often did when doing test prints. Wanna know how big a dust spot is when you are enlarging a segment of a negative with a theoretical size of 26 (8m), essentially you do touch up with a very small house paint brush designed for going around window frames; those were the days.
Unless you really critically align your enlarger with the mural wall, then you will have more of an issue than whether to use glass or glassless negative holders. In Way Beyond Monochrome, second edition, page 440, there is some discussion and some pictures of how they built (or someone built) a LASER alignment tool rather cheaply. I would suggest this could be a very practical and useful tool for aligning your negative carrier to the mural wall. Barring that, nothing wrong with using a scratched negative.
The best mural walls Ive ever used were covered in sheet steel and painted 18% grey, or something like that with a red cross line painted to show the centre and some horizontal and vertical lines to give you an idea for paper placement. One then used magnets to hold the paper in place. This is a very simple way of ensuring your paper is flat, or flat enough.
With regard to apertures, well almost everything we used was either f/5.6 f/5.6½ or f/8 and these were 360mm (I think) Apo Rodenstock stuff. There were a couple of 300 lenses, but for 8x10 they didnt cut it, too much fall off. A slightly longer than normal enlarging lens helps a real lot with edge fall off when you are stretching the friendship bit. For mural enlargements of 4x5 we used 180 Apo Rodenstocks, they were so sharp you could cut bread with them. By the way, I know you are doing B&W and generally an Apo lens is designed to be used for correcting the focus for the three colours into one point, they also make B&W prints look super crisp as well.
To obtain grain sharpness on your mural enlargements, you may find you will either need an assistant to move the focus adjustment, an electric motorised adjustment system, or do a lot of walking, whatever you do, take your time. With regard to focusing, Ive tried various methods, with the lens wide open, with the lens down to its working aperture
. I would suggest you focus with the lens wide open, white light and if using glassless, wait until the neg pops, if it does pop that is.
If your neg does pop, and Im thinking sometimes it takes 5 minutes or so to pop, and your exposure could be around that time. Then consider covering or hiding your enlarger light from the paper when you are doing an enlargement, wait for the neg to pop, then remover the cover and use a stop watch for the enlargements. It is that method I used very successfully over a long time doing mural enlargements with one of the dodgy enlargers we had. We had 14 darkrooms in our complex, by the way.
I think what you are doing is great, 20x enlargements are not that much of a big deal. The other day with a 35mm negative I did a sectional enlargement of the negative where I lifted the head up so I had a 1m wide image on the base board. Now 1,000mm divided by 36mm equals 27.78 times enlargement on my calculator. The prints were as crisp as anything.
The lenses we used in our darkrooms for 135 and 120 films were all optimised for 20x enlarging with some higher, 30x rings a bell. This 20x enlargement factor, was pretty much the minimum industry norm for professional work. The 4x5 and 8x10 lenses we used were optimised for slightly lower magnification from memory (could have been 25x and 18x for the two formats), but one still ended up with really big sharp crispy images.
Build what you think is right, it wont be, but it will possibly be 99% right, then you can make adjustments as and when you require. I really would like to see your set-up once you have it up and running, but that will never happen.
Some food for thought!
Mick.