Defining pine tar, and what it actually is, can be an important step in removing it from stone. Most people refer to pine tar or pine pitch as the sap from pine trees that drips from the trunks and branches. Manufactured pine tar, on the other hand, is usually much darker (black rather than brown), though just as viscous and sticky as pine sap. Extracted from pine wood by drying out wood and burning it, this pine tar is often used for projects such as roofing to help coat wooden shingles and such. Most techniques that will remove pine sap will also remove pine tar, but it's important to know the difference just in case.
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The way that pine tar or sap is removed from any material, stone included, is to use a cleaning agent that breaks it down and makes it easier to wipe away. There are a number of materials that will break down pine tar and sap that are fairly easy to procure as well. Gasoline, WD-40, paint cleaner and rubbing alcohol should all do the job easily enough. Just spray or pour the particular agent onto the pine tar and give it some time to work. Then wipe it away and vigorously scrub the area with hot, soapy water. Repeat this process if it's necessary.
There are certain factors that should be kept in mind when using this method to remove pine tar or pine sap from stone. For instance, if the removal agent (gasoline in particular) has a chance of staining the stone (such as a driveway or walkway made of concrete), then it's likely best that another chemical be used. Additionally, protection should be worn such as gloves and eye goggles to make sure that skin isn't damaged. Lastly, the area should be protected to make sure the chemical doesn't spill into plants, and if this is being done indoors, there should be proper ventilation.