The last voussoir. At this point, if the scaffolding comes out, the voussoirs would tumble to the earth.
Because the arch is not an arch until all pieces are in place, there is no way to know that it is really going to work until you have made all the pieces. Now it was time to see if my arch would work. I stacked up the voussoirs along the scaffold and placed the keystone on top. There was a gap of 1/2 inch on either side of the keystone, and that was exactly what I wanted, because the final structure would use mortar to enhance the frontward-backward stability, and that was just enough room for the mortar. I used wooden shims to fill up the space, then I was ready for the ultimate test: removing the scaffold. The scaffold was constructed so it could be gingerly coaxed out as the rocks settled in place, but to my gratification the rocks stayed put and the scaffold slid out with ease. Now standing free, I noted some rockiness and play in the arch, but that didn't concern me overly, because second of all, the final structure would be tighter with the mortar, and first of all, an arch is designed to hold weight. To test this nonchalance, I lifted an 80 lb rock and set it right down on the keystone, and sure enough, the play went away and the arch was absolutely solid.