A coreless induction furnace usually comprises a vessel having a refractory crucible inside a water cooled induction coil. The inner face of the induction coil is usually covered by a thin layer of refractory plaster which is called the coil grout. There is a need to interpose a layer between the coil grout and the refractory crucible to provide a slip plane between these two surfaces.

According to the invention in one aspect there is provided an induction furnace comprising a vessel comprising a refractory crucible within a cooled induction coil, slip plane material and reinforcing material being present between the crucible and the coil.

In a much preferred feature the slip plane material and the reinforcing material are present as a laminate.

Preferably the plane material is a flexible mica paper or sheet and the supporting material is a glass fabric. Preferably the glass fabric has a density of between about 500 gsm and about 1500 gsm, preferably 800 to 1350 gsm and a thickness of between 0.8 to about 2.5mm. preferably 1.4 mm to 2.2 mm. Preferably the glass fibre fabric is woven. Preferably the layers are coated and laminated with a high temperature inorganic binder.

In another aspect the invention provides a method of preparing an induction furnace comprising interposing slip plane material and a supporting material between the refractory crucible of the vessel and the surrounding cooled induction coil.

In a specific aspect the invention provides a method of preparing a coreless induction furnace, comprising interposing a laminate comprising a layer of flexible mica paper or sheet and a heavyweight layer of glass fabric between the refractory crucible and the surrounding cooled induction coil.

In yet another aspect the invention provides for use in the lining of an induction furnace, a laminate comprising a layer of flexible mica sheet and a layer of relatively heavyweight glass fabric.

In order that the invention may be well understood it will now be described by way of illustration only with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a vertical section through one water cooled coreless induction furnace of the invention; and Figure 2 is a side elevation of a laminate of the invention drawn to an enlarged scale. The furnace comprises a crucible 1 having a cast refractory side wall 2 and floor 3. A water cooled induction coil 4 surrounds the side wall 2. The coil is held to a metal frame, not shown, which is plastered by a layer of grout 5 to provide a smooth surface facing the crucible. The grout is about 8 to 10mm thick. Between the grout 5 and the side wall 2 is a laminate L of the invention.. As shown in Figure 2 the laminate comprises a high temperature flexible mica paper LI and a heavyweight woven glass fabric L2. The laminate is arranged so that the mica layer faces the coil grout. The mica paper is about 0.2 mm to about 0.35 mm thick and the glass fabric is about 0.8 to about 2.5 mm thick and has a density of about 500 to about 1500 gsm. The two layers are held together by a high temperature inorganic binder applied to the facing surfaces of the mica and the glass fibre fabric, and then urged under pressure on to the woven glass fabric. Mica, not shown, is usually applied to the floor 3.

In assembly the furnace laminate is fitted between the crucible wall and the coil grout and the floor. The laminate is provided as roll and cut to size and shape on site. The presence of the binder and the mica facing ensure that when cut any slithers of glass do not escape into the air and cause skin irritation to operatives. The presence of the laminate provides a good slip plane between the crucible and the coil grout. This allows a) the crucible to expand and contract during operation without either damaging the induction coil or allowing cracks to form in the crucible. b) The crucible to be broken out or pushed out much more easily when it is being replaced at the end of its useful life. Glass fibre fabric when used on its own often sticks to the coil grout. It can then only be removed by mechanical means thus generating a great deal of airborne glass fibres. The presence of the mica alongside the glass fabric produces a laminate with significantly improved heat transference characteristics than glass fibre fabric on its own. This has important technical advantages as it helps heat to be transferred away from the hot face of the crucible, towards the water cooled coil. This slows down the vitrification of the crucible and consequently enhances its service life.

The invention is not limited to the embodiment shown. Mica may be present on both sides of the glass fibre fabric. The glass fibre fabric may be non-woven.