The influence of the additives (10 wt.% rutile, anatase and zirconia) on the atomic-scale structure and microstructure of a base glaze (60 wt.% feldspar, 15 wt.% quartz, 8 wt.% kaolin, 12 wt.% dolomite and 5 wt.% zinc oxide) is investigated. The morphology and structure of the glazes were determined by Scanning Electron Microscopy, X-ray diffraction, infra-red spectroscopy, as well as Al-27 and Si-29 magic angle spinning (MAS) NMR. Microscopy and X-ray diffraction revealed that the opacity was caused by crystals formed during thermal treatment. Although there are clearly crystalline phases present NMR shows that the heat treated glazes are dominated by the glassy component meaning the opacity is largely conveyed by the added oxide. The relationship between composition, structure and thermal expansion coefficient of the glazes is examined.