For soft enamel pins, the enamel is laid in the recessed areas and put through ovens to bake the enamel hard. The enamel is only added once in this process which means when dry the enamel clings to the edges and recessed below the metal die line (that stops the enamels mixing). This give the soft enamel the 'bumpy' feel when you run your finger over the top. A clear epoxy resin coating can be added to protect the enamel and give the effect that the enamel is smooth, which can make the enamel appear flat but is not hard enamel.
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