Wednesday, 2 August 2017

Smooth Surfaces on the Bottom of Bowls

A frequently asked question is how to get a smooth shiny surface to the outside of slumped bowls. There are two certain ways – have the shape blown, or do a free drop. 

Avoid Moulds

In blown glass work the hot glass can be shaped in a cold mould, which means that the glass does not take up all the mould imperfections.  However, the glass must be put back into the glory hole to remove the chill marks from the cold mould.

A free drop is the process where the glass blank is placed over an opening which allows the glass to fall without touching any mould.  You need to observe periodically during the firing to arrest the drop when it is at the stage and shape you want.  You then need to remove and polish the rim that rested on the elevated ring that supported the glass during the drop.

Failing these techniques, you need to use a mould 

The surface of the glass that is in contact with the mould will take up the texture of the mould surface. When the glass is hot enough to take up the shape of the mould, it will be soft enough to take up some texture from the mould. The hotter you fire, the more texture will be imparted to the glass. 

You can minimise the texture of a mould 

Prepare the mould with the smoothest surface you can.  If the shape is simple enough, you can use very fine sandpaper - 6000 grit is useful.  This will give the smoothest possible mould surface.

Use the finest kiln wash you can find to coat the mould. The finer the powder is ground, the less texture is present.  You can also smooth the kiln washed surface with a balled-up piece of soft cloth or tights.  Do this very lightly, so that you do not rub off the kiln wash. Remove the excess powder before firing.

Minimise the temperature

A major way to reduce the texture is to fire at a slow rate to the lowest temperature you can, using a 30 to 90 minute soak. This will give you less texture than a fast rate to a higher temperature with a shorter soak. To determine how long is required at a low temperature , peek periodically to see if the slump is finished.

The principle is to fire as slowly and to as low a temperature as is practical.  This will reduce the chances of marking as long as the glass does not slip down steep sides.