Good luck!
Just make sure that while you are souring not to contaminate equipment you are using on other brews. There's a lot of wild stuff going on in there so be careful.
Too easy, just 4-5 pounds of wheat DME for a 5 gal batch. That should get you 1.030-1.040. Using DME will keep the wheat/barley ratio about 50/50. Do a 15 min or so boil, cool to 100 and toss in a handful of grain. Try to keep the temps around 100 until its where you want it. Boil for 30 min...
I don't ferment in there. But yeah, once it's as tart as I like it, I'll put the kettle on the burner with the chiller in it and boil the whole thing for 20-30 min. This should kill everything and the beer is now "clean". Then I'll just rack into a fermenter and treat it like any other beer...
I just set the oven just below warm and it will keep the wort between 90-100. I never considered the light. I'll have to try that next time.
With what you are proposing, you have to consider two things. If you pitch a commercial strain from white labs or wyeast, it will probably take months to...
I've got a dedicated BIAB sour wort Berliner system that works great!
I mash in a 5 gal boil kettle by BIAB. Then a cool to 100 after removing the bag. Now that its just wort, I toss in some whole grain and stick it in the oven for a few days to hold it around 90-100. Tasting it along the way...
This is what I do. Mash in boil kettle with BIAB, toss in a handful of grain, hold at around 100 for three or four days, wait until it is tart enough, quick boil, pitch yeast, wait a few days, keg and force carb. Quick, easy and clean.
A couple months seems pretty young still. It may take some time to develop character.
As far as rasberries, a little bit can go a long way with them, and it depends how much rasberry flavor you want in the finished product. I would start with maybe two pounds and add if you want more rasberry...
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My water comes from Salmon Springs in Sumner, which supplies about 50% of Puyallup. While this may not be your source, it should be pretty close. You could always treat using these numbers and check your ph after the mash has settled a bit, adjust as necessary.