Black clay that you find naturally is often black because of organic matter which will burn out when fired. The color could change to something brown or red after firing. One way you can test it is by making little cones and pinch pots which you fire first to bisque temperature, and then higher if it survives. You can also make a clay ruler to test for shrinkage and another piece to test the amount of water it absorbs after firing so you will know if it is vitrified.
To protect your shelves, make sure to fire the test pieces on a piece of scrap shelf or a pad of bisqued clay with raised sides. I have seen tests of these natural clays that have totally melted at high fire temperatures, but are great at lower temperatures. If it has a low melting temperature but a nice color when fired, you may be able to use it as a base for a slip or glaze.
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