Silica (SiO2) is widely used as sintering aid during vacuum sintering of YAG (Y3Al5O12)-based transparent ceramics. These ceramics are mainly used for laser applications when they are doped with rare-earth luminescent elements such as Yb3+ or Nd3+. By means of microstructural, chemical, dilatometry, and thermogravimetry analyses, this study has evidenced that sufficiently high amount of silica (ie above the solubility limit in YAG) forms intergranular transient liquid phase of mixed composition Y-Al-Si-O that vaporizes rapidly for temperatures higher than 1350 degrees C. As a result, silica content after sintering remains always lower than the solubility limit in YAG ceramics (ie lower than 900 ppm). Finally, vacuum sintering with an external source of gaseous Si was proven to be suitable to manufacture highly transparent Nd:YAG ceramics.