Those who produce tar soap in Sweden, don't use whatever tar they can find. It is a difference between factory made and handmade tar. The swedish producers use "dalbränd tjära" (direct translation: valley burnt tar). That is the traditional, handmade type, that is made outdoors. I don't know the english word for it, but I'm sure you have such tar in made in America as well. If not, Auson has it. And it can be ordered online. I don't know if Claessons has distributors in America, but Claessons also have good dalbränd tjära.
If Claessons is behind the company Wilma Naturprodukter or if they just cooperate with them, I don't know. But Wilma Naturprodukter makes tar salves and different things, and they use only dalbränd tjära. Wilma's products are linked to on Claessons website.
So I think it is important to use a tar that has traditionally been used to make tar soaps and creams. I don't know why, but in Sweden the soapmakers have a lot to of tar to choose from, and they only choose the valley burnt one. It must be for a reason.
I am reading a science report just now, in norwegian, about tar. That report analyzes and measures several types of tar. It is differences between them. The smell is different, for example, and the water content, volatile substances etc. But if the differences matters when it comes to soapmaking, I don't know. The report is about tar for our old wooden churches.
Here in Scandinavia, tar that is not specifically branded as "dalbränd" (swedish) or "milebrent" (norwegian) is factory made. The factory process uses higher temperatures in general, to get a higher yield. They also remove some of the substances. Factory made tar are in general darker than handmade.
The tar that you can find in veterinary shops in America, is most certain factory made, unless otherwise stated. I would not use that, because it is not used here. Especially the swedes and the finns have a long tradition of using tar in soaps, creams, mosquito repellent etc, etc. I have found several producers of such items, and I have not found that they use factory made tar. I have found a few that doesn't specify it.
I would find american traditionally handmade tar, or buy it from a swedish producer, like Auson or Claessons.
Personally, I was lucky to find local made tar. Here is a video of the making of the 2015 production. It is in norwegian, and he explaines how you build it and light it, and what not to do. You can see when it is set of fire, if you forward to 06.00 minutes:
http://nye-troms.no/nyheter/slik-ble-tjaremila-bygd-opp/19.3124
I'm 100% sure you have such production in America as well. So many scandinavians emigrating over the ocean in the old days, and some of their skills must be alive today. BUT, it is not important to find and use small scale made tar. I wanted to find it, because I felt like doing it. You can just buy tar made the exact same way, from Auson or Claessons (and some others as well). Maybe they make it in a bigger scale, but that doesn't matter.
If factory made and handmade tar did not differ chemically, it would probably not matter too much either. And maybe it doesn't matter.
But since tar is not a homogene product with exact the same content from batch to batch, from producer to producer, from which time the tar is extracted (early, mid or late in the burning process) etc, how can a saponification number be calculated? I don't trust the
lye calculators that give a saponification number for tar. PH varies, water content varies a lot, volatile substances varies, color, smell and taste varies etc.
I have found tar to most likely have a PH around 3 to 4. It does vary from 2 to 6 in the research paper I'm reading. But 3-4 seems the average for most of the samples. That is a low PH which will up the superfat if not taken into consideration.
I have not made any tar soap yet. I have not collected the tar I have just bought either. I'm just thinking loud about the PH and how to deal with it. I really don't know what to do in hot process, if the tar is added after the saponification? I don't think it is only the fatty acids in tar that lowers the PH, it contains other acids as well, vinegar acid, for example. I plan to saponify some of the tar, and add some after the cook as well, together with the superfat or drop superfat and just add tar as superfat.