I advise against using India Ink. You will thank me for it later. I have answered this question before in depth, but I will do so again for you.
India Ink is made of non-toxic materials for the most part, but is a soupy consistency that bleeds when you push it under the dermis, making crappy lines. It is so watery that you generally have to go over the line a few times to get it perfect, but even still, you will have spots that bleed out into the tissue around it, making it look really amateur and cheap. Not to mention that you will be scarring the guinea pig you have. Even if the ink goes in well, a lot of it tends to fall out during the healing process, and the overall hue of the ink that is left in the skin is going to be a dull black to a grayish color, or may even look sort of blue.
Tattoo ink is not hard at all to buy. It's not as though you must be a professional to procure it, but buy decent ink at least. I recomend that you get Kuro Sumi Ink, Intenze, or Starbrite Ink. They are all decently priced, and can be found on eBay. So keep the India Ink in the art supply store.
Chat with our AI personalities