I think I mixed things a little, I mixed tar and tar water. I couldn't find the paper I read once with detailed analysis of the tar's content. But, I found some information. And that is that the volume will shrink by 25% when the tar is boiled. It will get more viscous/thicker.
Boiling of tar is common in Norway for some uses. To preserve the wooden churches from the Viking age, the tar is boiled to achieve a more long term protection/a thicker barrier to protect the wood against the weather and sun. They experiment with different levels of boiling, different temperatures and time. But, boiling of tar is highly dangerous. It can boil over, catch fire and so on. And the temperature must be measured frequently. So maybe not a good idea to do at home. At least it must be outdoors. I guess a crockpot is well suited for boiling tar relatively safe.
The norm for kiln burned tar is that it should contain:
Specific weight: 1,03-1,07 g/ml
Volatile parts: 8-18%
Water soluble parts: 2-6%
It is not only tar that comes out of a kiln. It is three fractions that separate when the barrels are stored for some time: "Tjærelåg" (tar juice/water) that sinks to the bottom of the barrel. The tar in the middle and water on top. The bottom and top layers are removed.
How much tjærelåg and water the tar contains, will vary during the burning time of the kiln.
This is an example from a norwegian kiln:
Barrel no 1 (first fraction): 125 liter tar - 25 liter water and 30 liter tjærelåg
Barrel no 7 (medium fraction: 150 liter tar - 10 liter water - 15 liter tjærelåg
Barrel no 12 (last fraction): 150 liter tar - 0 liter water - 0 liter tjærelåg
The viscosity of the tar will change too. From thin in the beginning to thick at the end.
The high percentage of volatile parts in pine tar will get reduced or removed by boiling. Water soluble parts, well, I don't know what happens to them. It depends on what it is. I guess something can be water soluble and dispersed in oil, but not soluble in oil, like instant coffee. But kiln burned tar does have a water content, and that will evaporate of course when the tar is boiled.
And by boiling, I mean temperatures between 120 - 190 degrees celsius. That is what they use here, depending on what they are trying to achieve
I now remembered, the amount of volatile parts in pine tar is wrong. It had to do with some inaccurate testing methods. I just read it, and it is in one of the tabs I have closed. But it doesn't matter too much. The real number is up to 22% volatile substances. I can't remember the from number.
I have actually bought 3 liters of pine tar. But lazy me have not collected it yet, even if I bought it several months ago. I think I will try using it both "raw" and boiled in cold processed soap, to see if it makes any difference. But since it does make a difference on wooden church walls, it might do in soaps too.