Boiling the Bat
Fresh car battery acid (the Bat) is the main source of sulfuric acid by many. However, it is diluted sulfuric acid, and many experiments require concentrated. So you have to boil it down to concentrated. This process is best done in two steps.
Step first involves pouring the Bat in a flask and boiling it, preferably with boiling chips. For the most part, water will come out and sulfuric acid remain. However, this process can only go until the acid reaches the concentrations of 70-80%, when the noxious SOx gases and acid mist will start to emerge. If you have access to a fume hood, you can continue the process. Otherwise, you need a special distilling setup to distill remaining water off.
The setup must have two condensers: one air-cooled (such as the nose of the retort) and one water-cooled (such as a Liebig). The Liebig serves to condense the SOx gases and water vapor into dilute sulfuric acid, thwarting any possible damage done by them. The boiling flask should be insulated with asbestos cord, to prevent refluxing of sulfuric acid on the retort walls.
You pour your half-boiled (70-80%) sulfuric acid in the distillation retort, put in some boiling chips and assemble the apparatus. Once you start heating, you will notice white mist in the nose of the retort and droplets of very dilute sulfuric acid coming out as distillate. Ignore it.
The cue you should watch for is when oily waves of sulfuric acid appear on the inner walls of the retort nose instead of the dewey drops of water. From this on, you should check the strength of the distillate with any method. When it starts to char matches, it is a good sign: it means that at least 95% sulfuric acid is in the pot. If you can measure the density of the distillate, it is the best method: stop distilling when the distillate will start coming with a density of at least 1.8 g/cm3.
After that, turn off the apparatus, remove the asbestos and wait for it to cool. The liquid in the pot is sulfuric acid with a concentration of 96-98%, useful for most reactions.
Use only inert boiling chips such as silica gel. Chips like pieces of red brick tend to partially dissolve in the acid and contaminate it with ions of iron, making the acid ranging in color from castor oil to soy sauce. Avoid getting any organics into the pot: it will make the acid black. To clear your acid from organic blackness, add hydrogen peroxide in it and re-dry it as described above.