Priming with wort?

I generally keep several bottles of wort from my batches for starters, but wouldn't it be ideal to prime with them? Anyone every done it? How did you do the math?

Sure you can do it. Charlie Papazian in his book Joy of Homebrewing has a section dedicated to it. The main problem, if I recall, is figuring out how much of what gravity to use.

GUess I better go grab it then. It's around here somewhere

I believe the technique is called Krausening. Papazian has a nice section to calculate how many quarts of what density wort you need to prime a 5 gallon batch.

Let's say we have a gallon of beer to which we would normally add 1 oz corn sugar (which is about 46ppg and near-enough 100% fermentable). We need to add the same amount of fermentable to the fermented beer, as well as to the volume of the new wort. Let's say the priming wort is 1.040 and 70% fermentable.

46/16*.7 = we need about 2 gravity points per gallon, or 10 in total; we need a quart of 1.040 wort for 5 gallons. We need another twentieth of a quart for priming the primer, but let's round that to an ounce. We need 33oz of 1.040 wort to prime 5 gallons to the same level as 5oz of corn sugar.

Back of the envelope stuff....

That's the math i wanted right there. Thanks!