The Oldest Known Recipe I Could Find.
Believe it or not, this is a pretty old drink. The oldest recipe for it I could find is from the 1891 book “American Bar-Tender” by William Boothby of San Francisco, California. He gives three names for the same drink. He calls it a “Half and Half,” an “Arf an Arf,” and a “Black and Tan,” and all three are just half porter and half ale. He doesn’t specify exactly what kinds of ale or porter to use except for just ale and porter.
How to Layer Beer Step One: Gravity.
Layering is achieved by stacking different fluids based on gravity, from heaviest to lightest in order. By “gravity,” we mean the average atomic density of all the dissolved material and its molecules. Alcohol density is 789 kg/m³, water is 978 kg/m³, and sugar is 1586 kg/m³. The average gravity of an alcoholic drink is calculated based on the amount of these three combined items. As you can see, sugar is significantly the heaviest, water is in the middle, and alcohol is slightly lighter than water. So low sugar and high ABV spirits will have lower gravity than high sugar and low ABV liqueurs and thus will float on top.
Every different beer has a different gravity since they have different alcohol, water, and sugar combinations. So the trick is to either google each specific beer’s final gravity or experiment and physically try laying two beers. Thus not all Black and Tan are black on top and tan on the bottom. That all depends on the specific gravity of each beer (generally, there is a range for each type). Most pale ales are heavier than a porter like Guinness, but styles like IPA are often lighter than Guinness, which would result in a Black and Tan with the black on the bottom and the tan on top. To give a quick example, the gravity of Guinness is 1006 kg/m³, and the gravity of Sierra Nevada Pale Ale is 1.011 kg/m³; thus, the pale ale will sit at the bottom and the Guinness will float on top. Hell, even bud light is heavier than Guinness, but a beer like Asahi Super Dry has a final gravity of 1.005 kg/m³ so that it would sit on top (but barely). In my experience, though, Guinness is oddly lighter than anything else out there, and it’s the rare occurrence that any pale or brown ale is lighter than it, so it is usually safe just always to put Guinness on top.
Here is a handy chart on brewersfriend.com with the general range of gravities for each style of beer.
How to Layer Beer Step Two: Pouring.
Once you know the order, the next thing is the pour. Pour the first layer normal. I pour the first layer a little harder to try and agitate out some of the bubbles. Even though the first layer is heavier, the rising bubbles will force a bit of mixing and make the divide between the two beers less sharp. Once the bubbles of the first layer settle a bit, slowly pour over a bent spoon the second layer. This helps prevent the two layers from mixing and provides an excellent clean division.