overly dense breads

From my experience, this sort of thing can be a result of overproofing. If you're not getting the sort of lift you want before you put it in the oven, that could be a result of not kneading it enough. If it rises enough before you put it in the oven but doesn't give you any lift while in the oven, you might be overproofing or you might not be pulling it tight enough when you shape it.

When you score the loaf, does it bloom outward, remain open but not really stretch much, or begin to curl back in on itself? If it is curling back in on itself, it's definitely overproofed. If it blooms outward and swells, you're probably not proofing long enough. When you score the loaf you want it to remain open, maybe stretch a bit but not bloom outward and not shrink back in on itself.

These are just my personal experiences from working with loaves at home, others can very likely give different and more knowledgeable answers. What is your formula and process like? How long do you let it proof, at what temp, etc?

Could you post the recipe that exemplifies the problem you're having? Could you also post a brief description of your procedures? Could you also post how long each rise takes, and roughly how much the volume increases? Finally, could you post what temperature your oven is at and how long your bake takes?

And could you post a few pictures (baked crumb shot, maybe proofed loaf, maybe end of bulk rise, ...)?

There may be different reasons. As said above, it would be helpful to get a few more details. What kind of dough are you working with (wheat/rye/whole grain, etc)? Do the loafs have the desired shape when coming out of the oven or are they to flat for example? Do you have a rather cold or warm room? And so on.
Here are two aspects that from my personal experience are the most critical when trying to achieve an open and light crumb.

1. I think this is the most likely problem: The loafs were underproofed. This often results in breads with a dense crumb but some gigantic holes here and there. Proofing times can vary a lot with sourdough. If the recipe asks for 2 hours of final fermentation then you may need a whole 6 hours, if your room is cold and your starter is relatively young and weak. This also means that you need more time for the bulk rise and if you follow the recipe exactly the dough may contain almost no gas when being shaped and it takes quite a long time until it actually starts to visibly grow during the final rising.
Here is a good site which goes deeper into this topic: http://forums.egullet.org/index.php?/topic/82234-demo-proving-bread/ .

2. The gluten may be underdeveloped. Kneading times vary a lot, depending on whether you do it by hand or with a machine, the used ingredients (especially water content and fats), the use of an autolyse phase, the existance and amount of stretch & folds during the bulk fermentation, etc. The safest method is to do the windowpane test (the search engine here should give more information about this). You take a little bit of dough about the size of a walnut and carefully stretch it between your hands, holding it against light (a lamp, a window, ...). If you are able to stretch it so much that it becomes translucant without ripping, then it is good. It may still have some thicker veins, especially when you do stretch & folds to further develop the gluten, or when the dough contains whole grain or solid parts like seeds, but you should see the light through it.

Maybe the loaves weren't done baking. (My guess is this a somewhat separate problem rather than just another aspect of your problem with crumb density.)

The color and shape of the crust is a very indirect and quite poor indicator of whether or not the loaf is done baking. There are better ways.

One good way to judge if the loaf is done baking or not is to remove it from the oven (close the oven door and leave it on for the moment) and immediatly stick an "instant read" thermometer through an inconspicuous place on the crust deep into the interior of the bread. How done you want it is somewhat a matter of taste, but for regular bread at sea level the desired temperature is usually somewhere in the range 200F-210F. If the inside of the loaf hasn't reached the desired temperature yet, put it back in the oven. If on the other hand it's definitely done, turn the oven off.

(For thermometers, "instant read" means you poke it into whatever you're making, get a reading, and then remove it. It does not mean the thermometer is blazing fast. Unless you pay quite a bit of money for a very fast thermometer, you'll probably need to spend at least ten or fifteen seconds getting an accurate reading; just wait until the reading stops changing.)

If you're very lucky, the recipe will suggest what internal temperature to go for. More typically, you'll need to find out what temperature you like for that style of bread, then bake other similar loaves to that same temperature.

If you're not too sure about that method, some alternate methods of telling when a loaf is done baking are:

• the old "thump" test, where you rap sharply on the bottom with your knuckles and listen for a hollow sound (this isn't generally recommended because it's not very accurate at all, but it's better than nothing and better than following crust color)
• a "probe" oven thermometer that stays in the dough inside the oven the entire time it's baking, with a cable that comes out the closed oven door to a display unit on your countertop or stovetop
• figure out the right amount of time, then write it right on the recipe and use it for future loaves (this doesn't help the first time though:-)

(Also buy an inexpensive oven thermometer and find out what temperature your oven really is. It's not at all unusual for ovens to be "off" by 50F; there are typically procedures for calibrating the knob or display. Just be sure to check the thermometer several times, both with the element on and with it off, and use the middle temperature. In order not to go nuts turning the element on and off too quickly, ovens typically cycle from 25F below the desired temperature to 25F above the desired temperature. This is normal. If you look at the thermometer just once, you're likely to catch it at one extreme or the other of its cycle, and actually mess up your oven's temperature calibration rather than making it better.)

Me too. I've two guesses (and neither seems all that good:-).

First, maybe you're measuring the temperature fairly near the crust rather than sticking the thermometer deeply into the very center of the loaf. Especially beware that instant read thermometers often actually measure the temperature about an inch back from the end of their probe, not at the very end of their probe.

And second, maybe the "right temperature" isn't so right after all. What temperature a loaf is done at a) is rather subjective and b) can vary for different styles of bread.