This cone 6 black glaze looks glossy until placed beside the cone 04 one
The cone 6 one is on the left, it contains about 25% frit. Both are colored using a black stain. That low fire glaze on the right has a high percentage of frit, likely more than 80%, that is the main reason for the beautiful surface. Frits are really fantastic, and standard practice in industry. However potters have been slow to adopt them, thinking they are more expensive. But from a "total cost" viewpoint, they are cheaper.