Larger image here.
That's me in KV-21 in the Valley o' da Kings, probably in 1991. It's a pretty large, unfinished tomb. Some other photos at Ryan's site. It's pretty well built as you can see by the nice straight corners. I don't recall off the top of my head if it had already been plastered or not, but I seem to have a memory tickle that some plastering had already been done. They would plaster all of the walls to make a smooth surface to paint on.
The walls are nice white Theban limestone which is the upper formation in the VK. The many horizontal lines are chert/flint deposits which are ubiquitous in this formation and no doubt gave the builders fits, because it is much harder than the limestone. Much of it is lenticular like that, but there are also more rounded nodules.
What I am doing there is holding a plumb line along some mason's marks, the small round dots, which you can also see all over, particularly down a line near the corner. They're just small paint dots, probably made with just a finger dipped in paint. I don't know if we had a specific hypothesis in mind measuring the plumbedness of them. We mapped in an entire wall of the things in detail but to my knowledge nothing's been done with that data. They did seem to concentrate near corners so one would surmise that a more senior mason would check the plumb and squareness of corners and place his mark when a section was done adequately. Many did line up plumb like that, so one assumes they were using similar measurement tools.