Problems when Slumping

A range of problems appear in slumping.  These include bubbles, splits, puddling and more. Several causes are possible.  This blog looks at the problems, possible causes and remedies.

Bubbles

Blocked Vent Holes

 Absence of, or blocked holes at the bottom of the mould to allow air out into the kiln on all but shallow or cylindrical moulds can be a cause of bubbles. Prop the mould up on stilts if the hole does not go directly from under the glass and out of the side of the mould. Alternatively, drill a hole in the side to allow the air to escape from under the mould.

Wet moulds

In kiln forming, the moisture resulting from recently applied kiln wash is considered by some to be a cause of bubbles. The water in the mould will be evaporated by around 250°C/482°F in any sensible slumping schedule. At this temperature, the glass will not have begun to move, so the moisture can move out of the mould through any vent holes at the bottom of the mould, or past the glass as it rests on the edge of the mould.

The circumstance when a damp slumping mould could cause difficulties is when using an extremely fast rise of temperature. This is detrimental to the mould also, as the rapid formation of steam is more likely to break the mould rather than the glass. It is also unlikely to result in a good slump conforming to the mould without significant marking.

In casting with wet plaster/silica moulds water vapour can move toward the glass. Casting practice has alleviated some of the problem, by having an extended steam out before 200°C/395°F, or pouring the glass into the hot dry mould from a reservoir.

In pate de verre, the mould is most often packed while wet. The small particles normally allow any steaming of moisture to pass through, and so be dry at forming temperatures without blowing any bubbles.

Top Temperature

Bubbles at the bottom of the glass are much more likely to be the result of too high a process temperature if the previous two conditions are met. This high temperature allows the glass to slide down the mould.  The glass is not plastic enough to thicken and form a puddle at the bottom at most slumping temperatures. Instead, it begins to be pushed up from the lowest point due to the weight of the glass sliding down the sides.

 

Avoiding uprisings on the bottom of bowls.

Vent Holes

Make sure the holes are clear before placing the glass.

Wet Moulds

Ensure that the moulds are no more than damp before placing in the kiln.

Top Temperature

Firing for too long or at too high a temperature will cause the glass to continue sliding down. Having nowhere else to go, the bottom begins rising. This is the result of the weight of glass pressing down onto the bottom, especially on steep-sided moulds. This is a consistent experience across several kilns and with multiple users.

Low Slumping Temperatures.

Glass at low temperatures is affected largely by its weight and viscosity.

Viscosity Effects

Thick glass will fall more slowly than thin, when using the same schedule. Thick glass takes longer to equalise the upper and lower surface temperatures. Since the lower surface is stiffer (has a higher viscosity) it will move less using the same heat up rate. This means slower rates should be used, or a significant soak just above the strain point will be required. This softening of the glass evenly throughout the rise to the top temperature is critical in obtaining even slumps.

Splits in slumps

Without the slow progress to top temperature there can be problems. Sometimes the upper surface of the slump appears fine. It is the bottom that exhibits a split or tear that does not go all the way to the upper surface of the glass. It indicates the rate of advance was too - but only just - too fast to achieve the desired result.

 The ramp rate has been quick enough to get the top heated and become plastic. But the lower surface is still cold enough that it is brittle. The weight of the upper softened glass begins to push down before the bottom has become hot enough to be fully plastic. The force of the weight on the bottom can be enough to cause the glass to separate, rather than move as the surface does. This split on the bottom but not the top indicates a slower rate for that thickness is required. This shows the interaction between viscosity and weight.

 Sometimes the split is evident from the top. The cause of this kind of split is the same as a split on the bottom. But the ramp rate has been much faster in relation to the thickness or profile of the piece.

Weight

It is possible to have glass slightly overhang slumping moulds if you use low temperatures. The glass has the appearance of behaving differently at these low temperatures than at fusing temperatures.  

 

At low temperatures it cannot form exactly to the mould. It falls first in the middle. Because the glass is not very plastic, the edges rise up from the mould at first, because the weight there is not great enough to allow the unsupported glass to bend. The edges stay in line with the beginning of the bend in the middle.  

 

At the beginning of the slump the glass is not soft enough to stretch. It maintains its dimensions as it falls. For deep moulds, the glass moves progressively to move over the lip of the mould and begins to fall into the mould.

As the slump proceeds, the glass stretches very little and so the edges move further down the mould. The glass continues to slide down at the edges until the centre settles down onto the mould bottom. 

During this slide into place, the glass can become marked. This is usually most evident on back of the upper portions of the glass where most sliding is happening.

 With higher than necessary temperatures, the glass can continue to slide down the mould. Since the glass is still not fully plastic, the weight pushes the glass at the bottom upwards. This gives the appearance of a bubble, but is an uprising due to the pressure of the glass at the sides of the mould.

 

During the sliding of the glass along the mould, it becomes more marked. The marks often look like stretch marks. And in many senses, it is exactly that.

At higher temperatures or longer holds, the glass softens more. At this point the uprising collapses and the glass begins to thicken at the bottom. It also thins slightly at the top.

Remedies

Ramp Rates

The ramp rates should be slow.

• ·        This allows the glass to heat evenly throughout. This is important to get even slumps. 
•          Contrasting colours or a combination of opalescent and transparent glasses heat evenly with slow rates.
• ·        Slow rates allow glass with tack profiles to heat evenly.
• ·        It helps avoid splits in the bottom of slumped glass.
• ·        It allows lower slump temperature to be used.

Low Temperatures

Using the lowest practical slumping temperature gives the best results.

• ·        It allows glass with small overhangs of the mould to be successfully slumped.
• ·        Low temperature reduces the mould marks on the back of the glass.
• ·        Fewer stretch marks are in evidence.
• ·        Low slumping temperatures with long soaks reduce the uneven slump that is sometimes in evidence with deeper moulds.
• ·        Low temperatures allow different colours to heat more evenly.
• ·        Low temperatures reduce the thinning or thickening of glass in a high temperature slump.

More information is available here.

This information shows you need to keep the slumping temperature to the minimum required. To find out what that temperature is, watch the slumping in stages in brief peeks (do not stare!). Look at the piece for a second or two every five minutes before you reach your desired temperature and at intervals throughout the hold.

If it has slumped completely at the beginning of the hold, you are firing too high. Reduce your temperature in subsequent firings and watch in the same way to find what the required temperature and time is. There is absolutely no substitute in slumping but to watch by peeking to learn what your mould and glass require. 

What Temperature?

To determine the temperature needed for your piece, use slow ramp rates – between 100°C to 150°C/ 180°F to 270°F. Set your top temperature around 630°C/1170°F for a simple slump of fusing glass. For bottle or window glass you will need a temperature closer to 720°C/1330°F.

It is necessary to observe the progress of the slump as you do not know the best slumping temperature. Start watching the glass at about 10-minute intervals from about 600°C/1110°F. There is not much light in the kiln at this temperature, so an external light is useful. You can also observe the reflections of the elements on the glass. When the image of the elements begins to curve, you know the glass is beginning to bend. You then know that is the lowest possible slumping temperature when using that ramp rate.

Hold for at least 30 mins at the temperature when the glass begins to visibly drop. This may or may not be long enough. Continue checking at 5-10 minute intervals to know when the slump is complete. If the glass is completely slumped before the soak time is finished, advance to the next segment. If not fully slumped, you need to extend the soak time. These operations mean you need to know how to alter your schedule while firing. Consult your controller manual to learn how to do these things. Stop the hold when complete and advance to the anneal.

In some cases, you may need to increase temperature you set by 5-10°C. You can do this by scheduling a couple of segments with 10°C/18°F higher temperature each and 30 minute soaks each.  If you do not need them, you can skip them. If you do need the extra temperature, you have it scheduled already.  You will know if you need the extra segments by whether the glass has begun to curve at the start of the first of the soaks.  If it has not after 10 minutes, skip to the next segment. Once the new temperature has been reached, check for a curve in the glass. Again, if after 10 minutes there is no curve, skip to the next (higher temperature) segment.

A low temperature slump will allow the glass to conform to the shape of the mould without softening so much that it takes up all the markings of the mould. That in turn means there are spaces for the air to escape from under the glass all the way to the slumping temperature as well as through the air holes at the bottom. It also gives the most mark-free slump possible for your shape.

If you are slumping at such a temperature that the glass has sealed to the mould, you are firing too hot anyway. Or put more positively, use a low temperature slump, that is, a slump at the lowest temperature to achieve the desired result over an extended period of your choice.

More information is available in the eBook Low Temperature Kilnforming available through Etsy or Bullseye.