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Once these Pigmented Acrylic Inks dry they become permanent and waterproof. May try adding an acrylic medium or thinner, but this may change the consistency of the inks.

If you clean them immediately, and you can thin them with a bit of water if it seems too thick for the steel brush tool to handle.

A lot of inks just plain spoil if they are sitting for a long time, similar to milk souring. You could attempt to thin it (with water, since acrylic is water-based) and see if it helps.

Upon asking the people at Speedball this question, they told me it will show up on dark paper, but you may have to put on a second coat. We do have a marker that's called the Pilot gold and silver pens that are actually paint in a pen. We have them by Sharpie, as well (Sharpie paint pens). These will work on black paper, really opaque, but you may be wanting to brush it on, or use it in a pen of your own. Another trick some artists use is to mix gold gouache with a little water, to make an ink like consistency, and that is very opaque. A friend who was a calligrapher did this with her calligraphy pens.