This black one is on a cone 6 buff stoneware. Because a thin layer works well with this high-opacity engobe it is possible for the slurry to be more fluid, less gelled. An immediate benefit of this is that it dries much more quickly (enabling handling of the piece within a few hours). Another benefit is a much more even coverage than would be possible with more a viscous consistency. The thinner layer also and has much fewer issues with flaking during drying. However application of the engobe take patience, perhaps a minute waiting for drips to stop and gelling to set in to hold it in place.
Engobes are high-clay slurries that are applied to leather hard or dry ceramics and fire opaque. They are used for functional or decorative purposes.