After 2 years baking at home, and 7 months in a professional bakery, I have finally figured out how to consistently get an open crumb in a poolish baguette. The three things, IMHO, that I was doing wrong every time? Not having a hot enough stone, UNDER mixing, and over proofing.
1. Pre-heating --- after experimenting with preheating my stone on the bottom of the oven, and ending up with CHARRED baguette bottoms, but a nice irregular crumb, I realized that having a really really hot stone makes a big difference in developing the crumb structure. Just need to be careful that it's not toooo hot. I've found that preheating my stone on the oven floor (gas oven) for about 20-25 minutes at 470 F is pretty sufficient. Even this will slightly over cook the bottoms, though.
2. Mixing: I know, I know -- everywhere you read people say a short mix is the key to getting an open crumb. Well I spent 2 years using different lengths of the short, relying heavily on stretching and folding, and i never got much better than irregularly distributed holes, but still tight crumb structure (see picture 1 and 2).
Then, I got a Bosch mixer thinking this might improve things, but the normal Bosch bowl seemed to tear the dough, and didn't improve my results. Then I got the stainless steal bowl with the bottom-drive mixing arm. This also failed to change the result, using a "short mix". Bread turned out mostly the same as above
Then, I decided to try "over" mixing my dough to a full windowpane---which hammelman advises against for an improved mix . I let it run on speed 3 for 7-8 minutes (after a 20 minute autolyse, and 2 minutes on speed 1 to incorporate yeast and salt) and the dough came out beautifully developed: good strength, silky soft touch --- basically like butta'. This adjustment alone gave me a more irregular crumb. (I've found that mixing to this level still requires 3 folds to get to the right strength).
3. Over-proofing: Following the guidelines of certain established baguette recipes (e.g., Hammelman's), I was always left with a pretty gaseous dough -- which sounds desirable, but I always felt that I would lose most of it during shaping. I could try to shape lighter, and when I did I got a better crumb, but that meant my baguettes were too loose, which made cutting difficult and always gave a substandard aesthetic result. I started to get the hunch that having a gaseous dough could lead to excess loss of gas during shaping. I thought, maybe if I let it proof less, and the gluten was less stretched to its maximum capacity, the dough would be more likely to hold a higher percentage of its gas during shaping. So, I started proofing my dough substantially less. This led to a drastic improvement in shaping and ease of dough handling. However, there's a fine line, as too much under proofing leaves the dough dense (albeit with a nice irregular crumb structure). I haven't completely gotten the proofing time right, but suffice to say the time that works for me is more like 1.5-2 hrs, using 0.3% yeast.
One other thing I've noticed, but i'm not totally sure about, is the timing of the folds. I have found that my dough has a much better feel and shapeability when the folds (however many you need), all occur before a significant amount of gas develops in the dough. For me, this means that if I'm doing a 2 hour proof, all 3 of my folds need to happen before the 1 hour mark, and then no more after that. Folding the dough after it has a lot of gas seems to give it strength, but makes it difficult to handle/shape later. Not sure why.
Anyway, combining these three changes, I get an open crumb almost every time (sometimes better than others, though). Here are some different batches:
My crust / cutting / ears still leaves something to be desired, but I'll figure it out with some more experimentation.
