You may pierce the clay or insert an eye pin at this point. This is also a good time to carefully wipe or dab the exposed portion of the marble with a cloth or towel dipped in alcohol. The idea is the remove any fingerprints or other traces of clay that may have have smudged the glass.
Place into a cool oven and bring the temperature up. Cure as the clay manufacturer directs.
Turn off the oven and allow the piece to cool slowly. (The glass marble in the piece could crack if subjected to sudden temperature shifts, so it’s best to play it safe. If you wish, you may remove the pendant from the oven, but don’t dunk it in ice water.)
Once the piece is completely cool, you can pull out the eye pin, dab it with superglue, and put it back in place. If there are any rough spots or fingerprints you want to get rid of, then before you glue the pin back in place, use the wet-dry sandpaper as usual. Whether or not you decide to sand, you can use a soft denim rag to buff the clay into a gentle sheen. (Buffing with a power tool is not recommended, as the glass marble could crack if the buffing wheel were to grab and fling the piece.)
Congratulations! Your marble picture pendant is ready to string onto a necklace!