Unprimed linen canvas 8×8 warp and weft

Do you mean a fabric with 8 threads per centimeter in both the warp and weft directions (a coarse weave) or do you mean a fabric that can be ordered in 8 x 8 foot sections? I am assuming the former, and will take a look at my collection of supplier’s samplebooks when I return to my office on Monday

Sorry for the belated response. This has been harder to track down than I initially thought. Few providers of artist’s canvas list the thread count. I had to go and measure some that I have in sample books and stock used for my classes. Kremer appears to offer a 8 x 10 per cm. I have a roll of hemp canvas that measured about what you are looking for (8 x 8.5) I do not have the packaging to provide the exact type with total confidence but I believe that it is SoHo Urban Artist Professional Unprimed Canvas #25 Hemp. I can’t absolutely promise that is the brand, but I think so. The fabric is very coarse with a pronounce weave and varied texture. It would only really work for large paintings and even then would likely require a number of layers of ground to surmount the rather obtrusive texture.
There are likely other canvases that meet your criteria, it is just hard to know without personally measuring the thread count. I am sure that Utrecht had an appropriate canvas in the past but they seem to have stopped offering a range of their own unprimed linen.

If you are looking for stability and not specific surface texture, many artists’ grade linens will suffice. I would look for a tightly woven linen of a heavy weight rather than trying to find one that has a certain number of threads per CM. The two are not synonymous. Sometimes a low thread count is the result of a loose weave and not robustness, a loose open weave creates a poor substrate if it is not completely adhered to a rigid support.
While this is a different subject, large paintings on fabric really benefit from being stretched over a solid support, even if the fabric is not glued to the surface. Yes, this does add weight. Please read over our “Rigid Supports” documents in our Resources Section on the subject for more information.
https://www.artcons.udel.edu/mitra/resources

I am not sure that there are any hard and fast numbers for this and it certainly does involve a number of factors. For an oil painting on linen, I would first want to make sure that I was selecting the candidates from samples that exhibit a tight weave. From this group, I would then select the heaviest weight (oz or gms per standard size, eg 15 oz per square yard) that satisfied my chosen surface texture and that I could afford for the project.
If I had to ascribe an arbitrary number for a larger work in oil paint on a linen canvas, I would say that larger canvases (not stretched over, nor adhered to a rigid auxiliary support) should probably be no lighter than 12oz per square yard.  Again, this is an arbitrary number and does not take all factors into consideration (for instance the differences between a 3 x 3 foot painting and a six by six foot painting, the types of priming/ground, the stability of the stretcher system, the number of layers of ground, the thickness of the paint, the pigments used in the painting) You get my point.