Stencils vs. Saw

Saw

Frequently when people want to make a complicated shape they resort to a saw to create the shape.  This is used in both stained glass and fused glass work.  Although it may be necessary in stained glass applications, it is not as necessary in fusing.

One of a variety of saws

Stencils

There is an alternative to an expensive saw – stencils and frits.  You can make a stencil from stiff card. Place the stencil in the appropriate place. Then sift powder or sprinkle frit over the stencil.  Lift carefully and the shape is there ready for fusing.

Example of sifting powder over a complicated stencil

To get the depth of colour obtained from sheet glass, you need to apply the powder or frit to at least 2mm or 0.079". This also means that you need to go to a contour or full fuse with the powder or frit on the top surface.  You can, of course, later cap and fire again.

Example of the cutting of a stencil

More guidance on stencils is available here.

Revised 5.1.25

Mica

What it is

Mica is widely distributed throughout the world and occurs in igneous, metamorphic and sedimentary rocks. Mica is similar to granite in its crystalline composition.  The nearly perfect cleavage, which is the most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms.

Mica can be composed of a variety of minerals giving various colours and transparency. Purple, rosy, silver and grey colours come from the mineral called lepidolite.  

Dark green, brown and black come from biotite.  

Yellowish-brown, green and white come from phlogopite.  

Colourless and transparent micas are called muscovite.  

All these have a pearly vitreous lustre.

The melting point of mica depends on its exact composition, but ranges from 700⁰C to 1000⁰C.

Glass has a specific gravity of about 2.5, and mica ranges from 2.8-3.1, so it is slightly heavier than glass.

Tips on uses of mica powder and flakes

The naturally occurring colours are largely impervious to kiln forming temperatures.  Other added colours have various resistances to the heat of fusing. This is determined by the temperatures used to apply the colour to the mica.  Cosmetic mica is coloured at low temperatures and will not survive kiln forming with their colour in tact.

Mica does not combine with glass, but is encased by glass as it sinks into the glass surface.  You can use various fluxes to soften the surface of the glass.  Borax is one of those.  The cleaving of the mica results in only the layer in contact with the glass sticking.  The upper layers brush off.  This applies to both powder and flakes. One solution is to fire with mica on top in the initial firing and then cap for the final one.

When encasing mica exercise caution. Micas flakes must be applied thinly, as air is easily trapped between layers which leads to large bubbles from between layers of glass.  This is the result of the shearing of layers of the flakes allowing air between layers.  Although powdered mica is less likely to create large bubbles, air bubbles are often created for the same reason.  This is the reason it is most often recommended to fire the mica on top. 

Of course, one use of the mica to make complicated designs is to cover the whole area and fuse.  Then sandblast a design removing the mica from areas of the glass. You can then fire polish, or cap and re-fire to seal the mica.

Mica safety

MSDS for mica only mentions the inhalation of the dust as a risk. Mica is resistant to acid attack and is largely inert.  Inhalation of the dust is a (low level) risk.  Any significant health and safety problems relate to the coloured coatings.

Deep slumps

One of Karl Harron's deep slumped bowls

Deep slumps require multiple stages to get even drops without thinning the sides.  There are several makers of staged slumping moulds which allow progressively deeper slumps in a series of firings into deeper moulds.

If you have a steep-sided mould, you will find slumping directly into the shape difficult.  There will be uneven slumps, thinning of sides, hang-ups, etc., among your attempts to achieve the slump in one firing.  It is possible to mimic this series of moulds without buying the whole set. 

To avoid these difficulties, you can build up the inside bottom of the mould by placing powdered kiln wash in the bottom and smoothing it to a gentle curve. You should aim for a gentle shape as in a ball mould. 

After the first firing, remove some of the powder, placing it in a clean container.  Shape the remaining powder into a deeper slump than the first one. 

It takes some time and practice to achieve a smooth even curve.  You can ease the shaping process by cutting the intermediate shapes from stiff card.  This can be rotated to achieve an even curve in the powder.  Remove any excess powder and do a final rotation to give the powder a final smoothing.  Place the glass back on the mould and fire.

It may be that you will need to repeat this several times to get the full slump.  Separate template curves need to be cut for each slump if you are doing more than one intermediate slump. It does depend on the steepness of the mould sides and the depth of the slump as to how many stages are required.  Sometimes the slump can be achieved in only two stages.

After firing the powder, pour it back into your kiln wash container, as it still is good for mixing to apply to shelves, moulds etc.

This method is useful for any mould that is too deep for achieving the slump in one firing, and without buying intermediate moulds.  Remember the final result will be smaller than the size of the deep mould, as the span of the glass becomes less with each deeper slump.