Wednesday, 20 May 2020

Pin holes in melts

Pin holes in screen and pot melts are the result of very small bubbles rising to the surface.  These bubbles are sometimes within the glass melted, but more often come from small amounts of air trapped within the flowing glass.  These are perceived to be unsightly, or make it impractical to make a functional piece from the melt.

There are ways to minimise bubble formation or to deal with the formed bubbles.

Bubble Formation
In pot and screen melts, the glass spirals as it touches down onto the shelf. This spiralling action can trap small amounts of air as each successive spiral forms beside the previous one. Efforts at prevention of tiny bubbles in the final piece need to concentrate on this fact.

A preliminary element in bubble prevention is to have a long bubble squeeze to allow the glass to settle in the pot or on the screen so that the rest of the process can proceed with a minimum amount of air trapped within the flowing glass.

Two-Stage Drop
In some cases. it is possible to have the glass flow from the pot onto an angled shelf where the spiralling glass has to flow from the initial touch down to the edge and then flow onto the shelf.  This allows any tiny bubbles initially trapped to escape before the final drop onto the shelf.  This provides two mixing processes and means that a lot of clear glass needs to be included to avoid a complete mix of the colours.  It requires careful selection of the original colours to avoid a brown or black result.  It also requires a big kiln with sufficient height for a two stage drop.

This two-stage drop is of course, not suitable for a screen melt where you wish the glass strands to remain.  Nor is it suitable when you wish to have many “pools” of colour mix in the final piece.

Where the two-stage drop is not practical or suitable other methods can be used.  These relate to scheduling, cold working the surface and re-firing the piece.

Scheduling relates to using a soak at full fuse temperature before proceeding to the anneal.  The melt will occur at 850°C to 950°C.  You can cool as fast as possible to a full fuse temperature of about 810°C and soak there for an hour or more.  This allows the small bubbles to surface, break and heal.  Schedule the rapid cool to the annealing soak, once the high temperature soak is complete.  This will eliminate lots of the bubbles, but not all.

A sample friring schedule from bubble squeeze upwards and then down to a high temperature bubble reduction soak

Cold Work
Cold working the melt is about abrading the surface to open the bubbles that are just emerging to form a small dome at the surface.  Sand blasting is a common form, as usually kiln wash or fibre needs to be removed from the bottom of the melt, and some devitrification from the surface.  It would be possible to continue to grind the surface of the glass to eliminate the small depression in the glass caused by the now opened bubble, but this is likely to expose more bubbles that were at a slightly deeper level. What next?

As you will need to do a fire polish firing after blasting or grinding the surface, you can use a full fuse temperature to allow the surface to become plastic enough to fill the bubble holes.  Remember to schedule the firing as though the piece were at least 12mm thick.  You may find that more bubbles are exposed in addition to the ones healed at the conclusion of this second firing.

An alternative is to fire upside down.  You will have noted that there are no bubbles on the bottom of the melt.  This is because the bubbles have risen through the heated glass.  This physical fact can be used in the second firing.  Fire with the melt surface to the shelf.  It is best to have a clean and newly kiln washed shelf, or fibre paper (not Thinfire or Papyros) under the glass. Fire the glass to a full fuse or high temperature tack fuse with a significant length of soak to allow the bubbles near the original surface to move toward the interior of the glass.  After firing, the glass will need thorough cleaning before being fire polished. This should leave you with a pin hole free piece.

Achieving a pin hole free pot or screen melt requires several stages of coldworking and firing.  This makes melts inexpensive in materials (it is scrap of course) but expensive in time and firings.

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