Cased is the name given to glass where the body of the article is made up of layers of different coloured glass; the layers can be transparent or opaque.
Cased glass can be achieved by gathering one colour on top of another colour from two different pots; the results from this method are not very reliable and seldom used. To obtain a more accurate and consistent cased blank a cupping method was used. This entailed blowing a cup of the outer layer of glass, positioning it into a heated iron mould and then placing and blowing another gather of the inside colour into it. In this way the outer layer would adhere to the inner layer and the piece could be worked in the usual manner. The following video shows the manufacture of a cased blank using the cupping method:
Decorative casing effects
Other decorative effects can be achieved by including coloured threads in the outer layer as in Osiris, trapping air in a pattern between layers known as Air Trap or including different coloured crushed glass in the inner layer as with Moss Agate.
The outer layer can also be a shaded colour which is more intense at the top and dies away to the base or vice versa; this technique was given the name Nuancé by Stevens & Williams. To create a shaded outer casing, this entailed taking a gather of coloured glass and “blowing through the bubble”, which meant that the glass near the iron would be thick and the true colour but this would get weaker as the glass thinned out prior to the bubble bursting. A gather with the inner colour would be blown into the open ended bubble on the other iron, which would be broken off, and the article finished in the usual manner.
For an explanation and comparison of shaded and heat sensitive glass click here.