Spikes on Frit Castings

Credit: The Crucible.com

It is frequent to have castings from frit with spikes, needles, or prickles around the edges. 

Causes

These spikes result from the glass touching the edge of the mould or separator during the hottest part of the firing. The glass particles first begin to compact as the glass rises toward the fusing temperatures. As the temperature increases toward the casting temperature it begins to expand both horizontally and vertically from that compact mass. As it cools, the glass sinks down and retreats from the edge. This movement leaves some small bits of glass stuck to the sides. The glass contracts as it cools, leaving the spikes as it contracts from its hottest state. 

Avoidance

The usual recommendation is to mound frit in the middle and let it flow to the outside. Still, the glass flows to the outside of the mould at casting temperature and it touches the sides. Leaving the risk of creating spikes. Accurate measuring of the amount of glass to charge the mould with is important. With the right amount of glass, the mould will not be overfilled and so, reduce the spiking. 

Measuring the weight of glass for the mould is not difficult. In many cases, the manufacturer of the mould has done the work for you. If you need to calculate the weight of glass required for the mould, it is not difficult. A method is given here. In short, you use a dry fill of the mould. Measure the volume (using the metric system) and multiply by the specific gravity to get the weight in grams. 

Larger chunks of glass tend to produce fewer spikes than smaller frit. Usually longer soaks at top temperature are required to fully form the glass with smaller frit. It is also possible to drip glass into the mould from a pot suspended above the mould. Accurate measurement of the weight will still be important. But add 100gms/4oz. to the amount to allow for the glass that will stick to the pot.

My view is that with dams, it is better to use a straight sided shape with fibre cushioning around the outside. When annealed and cool, clean it well. Then fire polish with a slow ramp to 540°C/1000°F followed by a quick ramp to the fire polish temperature. This will polish the sides of the piece that were in contact with fibre paper.

Prevention of Spikes at Corners

Often, after fusing rectangles we are left with sharp points at the corners. How can we prevent it?

 

Nipping corners off rectangles, especially opalescent and the underlying base pieces, is a standard practice to avoid sharp points on corners of a finished piece.

The principles of this relate to how glass contracts when cooling.

The glass expands in the fusing and then contracts on cooling. The hot glass can attach to a rough bit on the paper or shelf and be stretched in the cooling. This results in a sharp needle point.  There is more glass at the corners than at the sides to contract. This leads to the creation of more sharp points at the corners.

Only a small portion of the corner needs to be nipped off as shown in the first picture. It will not affect the dimensions of the piece. It will not affect the appearance of the corners. Except, of course there will be no sharp points.

The fix is simple. Nip a bit of the corner off before assembly. Also no more than 10 minutes at top Temperature should be needed.

Full details are in my eBook Low Temperature Kilnforming.

 

Simultaneous Fusing and Slumping

“I sometimes slump at the same time as I do a tack fuse. Is slumping at this higher heat bad for the mould? “

Image credit: Creative Glass

Mould

 It is possibly not bad for the mould, but it does depend on your temperature and heat work.  Ceramic moulds are typically fired to 1200° or 1300°C so higher kilnforming temperatures are unlikely to affect the moulds.  The speed at which the target temperature is reached is of concern though.  Ceramics have what is called quartz inversions.

 Two of the constituents of ceramics – cristobalite and quartz – have significantly large expansions at 226°C and 570°C / 440°F and 1060°F.  Rapid rises through these two temperatures risks breaking the ceramic mould.  This is not the case with steel moulds, of course.

Glass

 There may also be effects on the glass.  Slumping typically ranges between 620°C to 677°C (1150°F to 1250°F).  Tack fusing typically is done in the 740°C to 790°C (1365°F to 1455°F)range.  This is a significant difference even at the higher end of the slumping range and the lower end of the tack fusing range. 

 Some of the effects are:

·        The marking of the slumped glass will be greater at tack fusing. 
·        The glass will slip down the mould more. 
·        Any pieces applied to the base are likely to slide during the slumping process.
·        There is a risk of creating an uprising or bubble at the bottom as the glass slips down the side of the mould. 
·        There is more risk of creating needle points at the edges.

 Performing two processes at the same time risks difficulties.  Inevitably, compromises will need to be made between slumping and tack fusing.  Eventually, it will come to a time when the two process won't work together.

  

A slump taken to tack fusing temperatures is at risk from uprisings at the bottom, needling at the edges, excessive marking on the back, slipping down the mould and thickening

Needle Points

Often fused glass has prickles or needle points around the edges and especially at corners after firing.

This illustration is from Glass Fusing Made Easy

The nature of glass and its interaction with the separators is the cause.  As you heat glass it expands. Once the cooling starts, the glass contracts. Often a particle of the glass sticks to the separator while the rest continues to contract. This dragging of the glass along the separator results in the creation of little sharp points developing as the glass retreats to its final dimensions.

The best solution I have found to reducing the points at corners is to blunt any points or corners before assembly. Only a tiny amount of glass needs to be removed from the corners to reduce the possibility of these points being developed.

Small needle points can also develop along the sides of the glass too.  These are more difficult to avoid.  The most successful method for me is to use a loose separator.  This can be Thinfire, Papyros or a fine dusting of alumina hydrate or powdered kiln wash.  Although less widely available, talc can be used. Talc is known to be carcinogenetic with high exposure, so breathing protection is needed. All these powders provide enough lubrication to allow the runny glass to slide without sticking. 

Of course, you can use boron nitride, which is very slippery, but the cost of it makes it expensive in comparison to the other methods, including using fine diamond pads to remove the needles.

An additional consideration is the temperature you use.  The higher the temperature, the more the expansion.  Expansion rates are almost exponential above the brittle phase of the glass.  Reducing the temperature by 20C and doubling time or more means the glass does not expand so much and the additional time allows the desired profile to be achieved.  

Of course, paying attention to volume control - using 6mm or more thickness - will help to reduce the needle points.  A 3mm sheet both expands and becomes thicker at the edges by drawing more glass from the interior and the edge while attempting to reach 6mm.  This means there is an increase in the needling effect.  Although a 6mm piece retreats on cooling, it does not have the additional thickening effect of a 3mm piece.  Even a 9mm piece retreats on cooling, although the final piece has a larger area than at the start. 

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There are various preventive measures that can be taken to avoid needle points on fused glass.  These range from altering the edges of the glass, using fibre papers that turn to powder, using refractory powders, or boron nitride. Post firing solutions relate to cold working.

Sharp points on rectangles

At the conclusion of firing pieces with right angles or sharper shapes you often find very sharp needle points at the corners.

This is a result of the expansion of the glass as it heats up.  At top temperature, the glass piece is larger on the shelf than when you put it in cold.  The amount of this expansion is related to the thickness of the piece and the temperature it has been fired at.

As the glass cools, it contracts.  The contraction at corners and points has greater effects on the glass than at the sides.  The corners are contracting from two sides rather than only one.  This makes them a little more resistant to contract and often leaves a little of the glass stuck at the furthermost point of expansion as it contracts.

I have found the best prevention of sharp points on the corners of rectangular pieces, and those with even sharper angles, is to nip off the tiniest bit of the corners. This very slight blunting of the corners allows the glass to expand and then retract without the corner or point catching on the separator and so creating the sharp needles.

Further information is available in the e-book: Low Temperature Kilnforming.

Needling in Bottle Moulds

Sometimes people experience sharp, needle-like points on the bottle after it is slumped.

Causes

As the bottle expands and softens, it conforms to the surface of the mould.  When the cooling begins, some parts of the glass are trapped in the tiny pits of unevenness that always exist in the mould or in the separator.  As the glass retreats, the glass is stretched until it breaks off, leaving the sharp needles.

Prevention

Remedies relate to separators and temperatures.  This of course, assumes you already have a good coating of kiln wash or similar separator on the mould.

Separators

These additional separators can be fibre paper or powders.  Thinfire laid on the bottom of a bottle mould can provide additional separation between the bottle and the glass.  This works, because with a slow rate of advance, the Thinfire will have turned to powder before the bottle begins to slump. This powder will not interfere with any designs on the mould.  Papyros will work on smooth moulds, but not so well with textured bottle moulds, because of its more fibrous nature.

This use of powered paper indicates that you could use a cheaper solution.  Just dust a fine film of kiln wash on the mould.  I do this by placing the powdered kiln wash in a sock and shake the sock above the mould.  This will allow an almost invisible layer of fine powder to fall onto the mould.  This is enough to provide an additional layer of separation between the glass and the mould.

Temperature

It is quite common for people to slump bottles at tack fusing temperatures to do both the flattening and the slumping at one firing. This is quite hard on the mould and softens the glass enough to promote the needling. 

It may be better to use two firings – one to flatten using tack fusing temperatures, and one to form the bottle at slumping temperature.  This lower temperature will avoid the needling, as the bottle will not soften enough to form the needles during the slumping. The reason many people avoid this is because the bottles tend to devitrify on second firings.  If you do this two-stage slumping, you will need to apply a devitrification solution to the upper surface of the flat bottle to try to prevent it.

You can take a different solution to the two-stage firing.  As lower temperatures reduce the possibility of needling, you can simply soak for a longer time at the slumping temperature than a normal one stage tack and slump.  You will need to peek in at intervals to determine when the slump is finished, of course.  After a few firings though, you will get a good idea of the amount of time required to complete the slump. An additional advantage is that at the lower temperatures, devitrification is less likely.

Sharp Corners on Fused Rectangles

Often single or two layered rectangular pieces have sharp corners on the top or at the bottom.

This is caused in the case of the single layer, 3mm or less, piece by the glass trying to pull up to 6-7mm. 

In the case of the two layer, ca. 6mm, piece the sharp area is just at the bottom rather than the top.

In both cases the sharp corners on square and rectangular items 3 or 6mm thick can be avoided by nipping a very little off the corners before firing.