This is one of my all-time favorite techniques to use on smooth surfaces like coated papers, acetate or plastic. Alcohol inks have a funny way of spreading out when they touch a non-porous surface - the ink moves like a little amoeba while it's still wet.
Then it dries so quickly in the air, that it leaves a hard line where it stops moving. You can manipulate the ink while it's moving and drying with quick tapping motions of your applicator, and it starts to look like marble. Adding a second or third color blend and/or removes the first color in a way that adds to the cool marbly effect.
Click here to learn more about this technique and my little treasure box project over on the Bella blog - part of my biweekly #craftyourstash series. Happy Crafting!