So, I know there were lots of things I could have done to repair the situation had I been prepared (lesson learned, I'll be having DME on hand from here on out), but I'm a little more interested in what might have caused the problem and how I might prevent it in future all grain brews. Anyway, here's the grain bill:
10lbs Maris Otter 3 lbs Rye 1 lb Crystal Rye 8 oz. Rice Hulls
Dough-in was at 167 degrees with 4.5 gallons, and I hit my target mash temp of 153 right on the dot. Held with a loss of one degree through the entire hour-long mash. It took three vorlauf to clear and then I drained the first runnings.
Something weird did happen here. At first, I thought I was stuck, when the wort stopped draining at maybe 2 gallons. Opened the tun cut the grain bed a bit and it really looked like my grain had absorbed most of the water. So, I figured, sparge time. I don't have any grades on my brew kettle, but I think I had maybe 2.5 gallons, tops.
Had my 4 gallons of sparge water at 175 and started batching in immediately. I stirred, let the sparge water sit for about fifteen minutes, and then began sparging. The second runnings were extremely light, way lighter than the original wort. I've had that happen before, but the color change was never quite this dramatic.
Anyway, sparge finished and I had about 6 gallons in the kettle. Probably should have measured gravity then (not that I could have done anything about it), but I started the boil.
After boil and cooling, I had 3.5 gallons of wort at 1.053. OG should have been 1.072 and I should have had 5.5 gallons post boil.
What in the world did I do wrong?