A method of forming fused quartz glass is provided. The method includes the steps of: (a) providing a starting body made of fused quartz glass; (b) positioning the fused quartz glass starting body on a base plate; (c) inserting a first insert device into an interior cavity of the starting body to form an assembled structure; (d) heating the assembled structure to a predetermined temperature at which the fused quartz glass has a viscosity in a range of 105 to 1013 poise; and (e) deforming the fused quartz glass of the starting body at the predetermined temperature or in the viscosity range of 105 to 1013 poise around the first insert device to change a shape of the starting body. A method for making a large fused quartz glass vessel and a forming assembly for forming fused quartz glass are also provided.