Friday, 30 April 2010

Rapid Heat Rises and Their Effects on Firings

Based on a communication from Phil Hoppes

A word of caution. Never use 9999 for a ramp up. Note: 9999 just means on an up ramp the elements are full on, no cycling. On the down ramp the power is completely off until the desired temperature is reached. Your kiln will rise in temperature limited by 2 things - the type of insulation and the number of elements. This can be anywhere from 300 – 450C/hr. to as high as 1600C/hr.)

If the time it takes to go from your lower temp to your upper temp is less than 40 minutes, your controller will be unable to accurately control the top temperature. For example, if you want to ramp from room temperature (20C) to 300C and for your kiln 9999 on an up ramp is 850C/hr., the temperature rise you are looking to accomplish is 280C and your kiln will reach 300C in just under 20 minutes. The problem is that most controllers need around 40 minutes in any ramp cycle to "learn" how the kiln is responding to the inputs that are given to it by the controller. Slower ramps need less “learning” time, faster ramps need more time.

What will happen if you programme a ramp shorter than your controller will respond to is that the temperature in your kiln will not stop nicely at the programmed 300C. The controller has not learned how to stop your kiln from rising in temperature yet and the temperature will rise much higher than your programmed value.

Depending on your kiln and your controller this can be quite significant. Most controllers have a peak shut off value, somewhere between 55C and 85C above your programmed amount. Some controllers allow you to program this value also. If the temperature in your kiln overshoots the value it was programmed to stop and the amount of overshoot exceeds the programmed shutoff temp your controller will shut down. This is a safety feature and the controller is doing what it is suppose to do. If you have something in your kiln however and this happens it will not be annealed properly and you will have to very carefully re-fire to remove the stress or it will break into pieces.

It is a good idea to know just what your kiln will do. You can do this by taking an empty kiln, program 9999 in an up ramp from room temp to 815C. This is the typical peak you would use in a full fuse. See how long it takes for your kiln to reach this temp. This will give you the maximum up ramp rate of your kiln. You can use this rate to calculate if you violate the “learning” margin of the controller. 

It is advisable not to exceed 350C/hr up ramp unless overshooting the top temperature does not matter.

The 9999 ramp in almost all cases will be used to go from the top temperature to the start of the annealing cycle.

Monday, 26 April 2010

Prevention of Needling in Dammed/ Box Cast Work

To avoid needling in box cast or dammed work you need to provide a space for the glass to flow into.

This is done by using 3mm thick fibre paper to line the damming materials. The fibre paper is cut to 3mm less than the finished height of the fired piece.

Fire the glass with a long bubble soak. This allows the glass to almost achieve its final height before it becomes less viscous. It will still be higher than the fibre paper and as the glass continues to be more “soft” it will round as it reaches full fusing temperature. There is not enough glass above the fibre – only 3mm – for the glass to run over the fibre, as the surface tension holds it in until 6 or 7mm above the fibre. The top edge of the glass does not touch the fibre or dam, so there are no needles.

Another way to avoid needles in this kind of work is to make the dams larger than the glass being contained. That is, place the dams a short distance away from the glass. The glass will then flow out to meet the dams. Since the glass is not contracting it will not have needles. This is a good solution when the thickness of the glass is not critical. You control the area of the piece by the placing of the dams.

Thursday, 22 April 2010

Pot Melt Schedule

I usually use a schedule like this for either S96 or Bullseye:

100C/hr to 220C for 20 minutes; this is approximately the crystobalite inversion temperature – to be kind to the pot
220C/hr to 570C for 20 minutes; this is approximately the quartz inversion temperature – again to be kind to the pot
220C/hr to 677 for 30 minutes; this is a bubble squeeze temperature to allow larger bubbles to escape from the pot before melting begins.
330C/hr to 850C for 120 minutes; this is to ensure there is plenty of time to empty pot
AFAP to 805C for 30 minutes; this is to allow thickness equalization and also to allow bubbles to pop and seal.
AFAP to 482 for 90 minutes; this is for Bullseye, but is applicable to other glasses too.
55C/hr to 427C no soak;
99C/hr to 370C no soak;
120C/hr to 150 end;
Allow to cool to room temperature

Wednesday, 14 April 2010

Pot Melt Temperature Effects

When firing a pot melt, you have to consider how high a temperature you wish to use.

Viscosity is reduced with higher temperatures so increasing the flow and reducing the length of soak, although there are often some undesirable opacifying effects.

The size of the hole is also relevant to the temp chosen. The smaller the hole the higher the temperature will have to be to empty the pot in the same amount of time. Of course, you can just soak for longer at a lower temperature to achieve the desired object of emptying of the pot.

Using the same principle, the larger the hole the lower the temperature required to empty the pot in a given amount of time.

The temperature used to empty the pot will need to be between 840C and 925C. The problem with temperatures in the 900C to 925C range is that the hot colours tend to change, e.g., red opal tends to turn dark and sometimes brown. Some transparent glasses also opacify. So the best results seem to come from temperatures in the 840 to 850C range with longer soaks than would be required at 925C.

Saturday, 10 April 2010

Charging the Pot for a Melt

The way you charge (load the glass into) the pot makes a difference to the resulting piece.
A good way to get strong colour separation is to put two colours on opposite sides and a third colour or clear between them. The two side colours will have best separation if they are not more than 1/3 each. As the glass begins to flow out of the pot, all three colours will come out at once and form concentric circles (assuming a circular hole in the pot).

You can manipulate and alter the results with a fair amount of predictability by changing the diameter and shape of the hole, charging the pot with more than three colours or less, rearranging the orientation into a sunburst orientation or whatever comes to mind. Be sure to keep notes on what you did and what the results were in case you want to reproduce the effect.

Think about how the glass will flow out of the pot when you charge it with glass. If you layer colours horizontally from C (on top) to A (on bottom), it will initially flow out in colour A, then B, then C. After that initial flow, which will be on the outside of the finished piece, the main flow will be from the top (C), then the middle (B) and finally the bottom (A). This is because after the initial flow, the rest of the glass comes out in a funnel shape pulling the top and small portions of the underlying glass.

This means that layering is the best way of mixing colours. You need to think about colour combinations too. For example yellow and red become brown; yellow and blue a dark green, etc.

The proportion of dark colours is important, for example, as little as 2% of black can make the whole piece very dark. If you have dark colours, you need to add a large proportion of clear or very light opalescent glass.

If you use frit, large pieces are better than smaller ones. Even so, you need to be careful about the colours you use so the whole does not become muddy.

Tuesday, 6 April 2010

Aperture Pours

The most commonly used aperture pours are Pot melts and wire melts. Pot melts use containers, and wire mesh for wire melts. In both cases they control the way the glass melts into a container or directly on the shelf below.

The materials are stainless steel wire grids, and unglazed terracotta pots. The spacing of the steel grid will determine the number of trails of glass falling. So a finer grid will give more points of expansion in the resulting melt. But will mix the colours much more thoroughly than a coarser mesh will.

Doing a pot melt usually provides a simpler pattern of flow. A single round hole gives one circular point from which the glass expands. A single rectangular hole gives a single ribbon shape as the expansion point. You can, of course, have multiple holes in the bottom of the pot to provide a more complex interaction of the flowing glass. The wider the rim of the pot in relation to its depth, the more flexible it will be. You can put more glass in the pot and you can have it higher in the kiln.

The arrangement of glass in the pot will produce different results. There are two basic arrangements: colours layered one above each other as in a layer cake; and colours arranged on end around the sides of the pot. When loading the pot you need to remember that although the glass immediately above the hole will be the first to come out – and therefore be at the edge of the melt – the remainder of the glass comes out in a funnel-like order, with the glass at the bottom corner of the pot being the last to flow out – and become the centre of the melt.

There is a relationship between the hole size and distance to surface that affects the final appearance. The larger the hole the less likely the glass is to spiral as it falls, so you need a greater distance between the bottom of the pot and the shelf. The smaller the hole, the less distance you need. Only experience will tell you what distance and size you need or can use.

You can calculate the amount of glass for different sizes by using this table. If you have a rectangular space you are dropping into, you can calculate the volume of glass by multiplying the width, length and desired thickness – all in centimetres. This will give the volume in cubic centimetres and to convert that into weight, you multiply the volume by the specific gravity of glass - 2.5 is near enough – to get the number of grams of glass required. To convert into kilograms, divide by 1000.

By dropping directly onto kiln washed shelf, ring or circular container you will get some contamination.  There are some ways to avoid this given here.

You can also use this method to act as a crucible to pour glass into closed moulds.

Friday, 2 April 2010

Slumping unknown glasses

I had a recent request for help from an old friend who has taken up kiln formed glass. The problem is common enough, that (with her permission) I am adding it to the tips section.

I tried an experiment today to use some of my nice (non-fusing) glass. I cut at 270 mm diameter circle from a 3mm thick sheet and wanted to slump it into my 270 mm bowl mold. I set the mold up carefully and checked it was dead level in all directions and that the glass was absolutely centered on it. I have no idea what the COE is so decided just to use the S96 recommended slumping temperature of 650C. When I checked the kiln no more than 2 minutes after it had reached 665C, the glass had slumped almost to the bottom of the mold but it had slumped very asymmetrically. There was also a small burp on one side which has never been an issue when slumping bowls in this mold before.

The schedule I used was as follows:
200C/hour to 540C, 0 hold
650C/hour to 665C, 10 hold
Then standard S96 anneal programme

Also, the edges were still a bit rough from the cutting, i.e., they hadn‘t fire polished at all. Can you help?

Finding out about the softening characteristics of the glass
Slumping a single layer of glass with unknown characteristics – the CoE is not really relevant – requires that you watch it and other similar ones until you have established a slump temperature for the glass.

There is a way to do it:
Cut a piece of glass 305mm long by 20mm wide. Support it 25mm above the kiln shelf with the posts being 290mm apart. Put kiln furniture on top of the glass where it is supported. Make sure you can see the shelf just under the middle of the suspended glass when you are setting up this test. You can put a piece of wire or other dark element there on the floor of the kiln to help you see when the glass touches down.

Set the kiln to fire at 100C/hour to about 680C. Peek at the suspended glass every 5-10 minutes after 560C to see when the glass begins to move. Then watch more frequently. If your kiln has an alert mode on it, you could set it to ring at each 5C increase in temperature, otherwise use an alarm that has a snooze function to make sure you keep looking. When the glass touches down, record the temperature. This will approximate the slumping temperature in a simple curve mould.

Getting smooth edges
You need to have smooth edges before slumping. You can fire polish the piece of glass to get rounded edges, or you can cold work the edges with diamond hand pads, working from the roughest to the finest you have available. If that does not give you the edge you want, you will need to fire polish before you try to slump.

You can do at least two things to find the fire polish temperature. You can do a little experiment by using the cut off pieces of the glass and roughing them up a little before putting in the kiln. Make sure you can see it through the peep hole(s). Set the kiln to fire at about 250C to say 750C. Look in from about 700C to determine when the edges begin to round.

The other is to put a strip in with the slump test and set the kiln to go up to 750 rather than just 680. You can check on progress just as for the separate firing to determine the fire polish temperature. I think about 40C above slump temperature should be enough, but your test will determine that.

Avoiding uneven slumps
Most uneven slumps occur because of too fast a rate of increase in temperature. The piece can hang up on the mould sometimes causing the glass elsewhere to slide down to compensate. The real difficulty in the schedule was the 650C/hour rate up to the top temperature. This was so fast that the glass at the edges would have the opportunity to soften and so hang before the centre was soft enough to begin to bend. 150C or 200C/ hour would be fast enough from 540C to achieve the slump.

Other things can be done too. You mentioned the edges were rough from the cutting. This can cause difficulties of hanging. To avoid that, you should smooth the edges before placing the glass on the mould. A further precaution against uneven slumping is to give a slight bevel to the bottom edge so that it can slip more easily along the mould.

You had already done the leveling of the mould and the centralization of the glass on the mould – two other things that can cause uneven slumps.

Avoiding “burps”
The glass slipping a long way down the mould is often accompanied with burps or bubble like upwellings. These are both indications of too high a temperature. I would begin looking at the glass from about 600C in the slumping of any unknown piece of glass. That would apply to any new configuration of the glass or mould too. The fact that the glass slid to the bottom and had a burp means that the temperature was too high and too fast. Once you have established the lowest slumping temperature, by watching to see when it begins, you then can add about 30 minutes soak to that temperature. The length of this soak will have to be determined by observation and experience, though.

A slow heating allows the glass to be at an even temperature throughout its thickness. A rapid rise with a thick piece will sometimes reveal a tear like opening on the underside of the glass that does not come through to the top. This is because the upper surface is sufficiently hot to begin slumping while the bottom is just a little too cool. If there is too great a difference, the glass just breaks all the way through.

Also slow heating allows the slump to be accomplished at a lower temperature, leading to fewer problems and to less texture being taken up from the mould.