There is no use of making washable ink, if the pad is not new. It is almost impossible to remove all the ink from a pad, so one must not pour new washable ink into the old pad.
Even the new washable ink will turn into a rigid one after mixing with the previous ink. Therefore, you must also make a new ink pad. It is not hard by any means.
First of all, cut a sponge. Make sure the size is not large, because it must fit into the container having a lid. You can remove all the old sponge from the container and use it as a new pad. After getting rid of the old sponge, wash the container properly, making sure that there is no ink present inside.
After cutting the sponge, place it inside the container. You may use a little glue in order to prevent the sponge from moving inside the container. You pad is ready and now you need to work on making the ink.
Take a medium bowl and mix together four tablespoons of poster paint and one tablespoon of white school glue. After mixing it properly, put the material onto the sponge. Spread the liquid evenly on the sponge. Cover and let the sponge absorb the ink.
You can create washable ink of as many colours as you want. But make sure the container should be covered properly. If the ink dries out, you can add a few drops of water to make it alive again.
This ink will not stay on your clothes, because the poster paint is easily washable. One can also use this ink for rubber stamps.