Heat balsamic vinegar in a saucepan and bring to a simmer. Then, slowly stir in the sugar and salt. Continue to cook at a simmer for 15-20 minutes and stir every so often. Once the mixture has thickened and is reduced by half, the glaze is ready. Remove from heat and let cool before using. The glaze should be stored in an airtight container in the refrigerator. It will remain fresh for up to two weeks when stored this way.