As long as you don’t move the vase much you really wouldn’t need a barrier. If you absolutely wants one you could try making a disk that matches the size and shape of the layer line and glue that colored sand on it. If you coat it well enough and it doesn’t darken from the glue, that could look invisible.
Typically they don't mix as long as you leave it alone. If you really want to go the scientific route, finest sand on the bottom, coarsest on top, that way gravity won't move finer fractions into the coarser ones.
You don’t need a buffer just pour them in carefully in the desired pattern. You can also tilt the vase while pouring it in to get layers in angles. I always tap it just a little bit to help settle each layer before pouring the next onto it that way it ensures that the layer levels out some kind of like cake batter in a cake pan. If you are doing your own colored sand you have to make sure that it is very very dry before you attempt to layer it otherwise the colors will bleed.
ETA. If you want one color to run into the other like a design you would just take a thin object like a kebab skewer and poke it through one layer into the color below it along the perimeter of the inside of the vase. This affect can look really cool and is fun.
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JerseyWiseguy t1_isqft6m wrote
Typically, you just pour one atop the other--no barrier between them. You just have to make sure you don't shake it wildly, after you're done.