We made a brown ale yesterday. Well... more accurately, we tried to make a brown ale. It turned out a little darker than anticipated. Okay, well, a lot darker. We're considering calling it the "Black Brown Ale." Needless to say, color isn't really our thing.
Gravity, however, is. For those of you not in the Know, a beer's "specific gravity" is a measurement of the beer's density. It is expressed as a ratio of the density of the beer to the density of water. Since the density of water is defined to be one, brewer's typically refer to just the beer number. So you'll hear things like, "This pale has a gravity of 1.045." Or occasionally, just the latter most significant digits: "This pale has a gravity of 45." Gravities have a large range. Lighter beers, such as American Lagers, might have very low densities -- as low as 1.010 (or even lower!), while mealy Scottish stouts might have a densities of 1.080 or more.
When you're designing a beer recipe, the target gravity is one of the core things that you'll need to decide on. Typically, you determine this value based on how you like your beer and what kind of style you want to make. Lighter ales and lagers tend to have lower gravities -- around 1.015 to 1.035. The brown ale we made, and English pales, have slightly higher densities -- typically in the 1.045 to 1.055 range.
Not having ever made a brown ale from grain, we didn't really know what we were looking for in the finished product, so we chose a target gravity of 1.050. This is the gravity that you are trying to achieve coming straight out of the finished (cooled!) boil and into the primary fermenter.
The target gravity, combined with the desired final volume of beer, determines the amount of grain that you will need. Calculating the malt bill (deciding how much of various malts that you will need) is another post entirely, so we'll leave it for later. For now, just understand what gravity is, and how it affects the character of the final beer.
So now you kind of have to understand that a lot of brewing is incredibly inaccurate. You do these calculations using the target gravity and target volume of beer that you want, and then you proceed through the brewing process, kind of hacking your way through, estimating the compensating constantly.
Yesterday, we measured our gravity twice throughout the process. The first time was straight out of the boiling kettle after sparging. We had approximately (see? There's that word!) 10 gallons of wort, though our target was 11 total gallons of beer. The gravity was slightly high -- 1.054. Still fine for a brown, but a little more than we wanted. Note also: The wort, at this point, looked less like a brown and more like a porter.
We added another half-gallon of wort out of the sparge tank/mash tun, which at this point was fairly light on the gravity side. We then added a half-gallon of water , bringing the total amount in the kettle to 11 gallons. Assuming 10% boil off, we added another gallon of water and started the boil. At this point, we should have taken another gravity reading, but didn't because we were trying to finish the Blonde Ale... and we all a little... umm... yes.
After the boil and the chill, we filled one 5-gallon carboy, took a gravity, and then filled the second 5 gallon carboy. The gravity measurement was spot-on -- 1.050. It might be helpful to understand what a feat this was: We are all a little drunk from trying to finish the Blonde Ale. As yet, we have no accurate gallon-measuring device (so when I say "we had 10 gallons," what I mean is, "Yeah, that looks like about 10 gallons."). We took only two gravity measurements. But in the end, all our fudging ended up being incredibly accurate.
Well done, us.