Glass Stuck to Moulds

Glass that is stuck to moulds needs more care in removal than removal of glass from kiln shelves does.

The major element in removal is to get the mould to release the glass. This requires some diagnosis of why the glass is stuck.

• Is the glass trapping the mould? This happens most often when the glass is draped, especially over ceramic moulds.
• Has the glass been fired high enough to fuse to the mould? If you have fired the glass to tack fusing temperatures, you may find more occasions when the glass sticks slightly or firmly to the mould.
• Is the mould trapping the glass? This can happen when slumping into a steep sided steel mould. Occasionally a steep sided ceramic mould will show the same effect.
• Has the separator been too thin or failed? If none of the previous elements apply, it may be that the separator was too thin or has been fired to tack fusing temperatures in a previous firing.

You can use mechanical methods to free the glass from the mould by inserting a thin pallet knife between the glass and the mould. This works better for items where the glass is inside the mould. If the glass is outside the mould, the chances are that you will break the glass. Using mechanical methods for any glass that is more than lightly stuck will most often lead to breakage of the glass, and often both items.

Where the mould is trapping the glass, you can put the item back into the kiln, but upside down and supported a centimetre or so above the kiln shelf. Heat the glass gently toward slumping temperatures. The glass should fall from the mould at 300 or 400C, but you need to keep watch to make sure you do not over heat the glass.

Where the glass is trapping the mould and you are using mechanical methods, you normally need to decide which is more important – the mould or the glass. It is just possible to break a ceramic mould and leave the glass, if the glass is 6mm or more. If the mould is more indestructible, you will probably lose the glass.

With ceramic moulds you can try heating the two to 300C or 400C and reach in with appropriate protection to try to lift the mould out of the glass. If the mould is steel, it will expand faster than the glass and break it.

Once you have the glass off, you may need to repair the mould.

Aperture Drops Finishing

After the piece has cooled and been removed from its ring, you can consider how to finish the piece.  The first decision is whether to retain or remove the rim from the vessel. In some cases, the rim can be retained as an integral part of the piece and there is little work needed to finish the piece.  Possibly only tidying up the edge of the rim and cleaning the bottom.

Removing the rim

But for most aperture drops and for most people, it is desirable to remove the rim. To have successful drops without rims, you most often need to have access to cutting and polishing equipment.  There are several ways to do this. 

The method that uses least equipment is to score around the upside down drop just above the rim.  When scored, tap the rim with a soft hammer to release it.  This is not always an even break and sometimes runs into the length of the drop.

A low tech way of cutting is to put a diamond cutting blade on a Dremel-like battery powered tool and with a flow of water grind through the side of the drop.  It is best to have a small flow of water directed at the cutting area, rather than immersing the rim in a bath of water.  This helps avoid electrical shock.

The rim can be cut off in portions with a tile saw, cutting quarters, eighths, sixteenths off the rim, approaching the edge of the drop.  Those with adjustable height wet saws can cut through small portions at a time of the rim, and support both the rim and the drop, especially when nearing the completion of the cuts.

There are also specialised versions of the wet angle grinder that make cutting of the rim easy and much more certain of a good result.

Finishing

After any of these methods of removing the rim, the drop edge, and possibly bottom, needs to be ground and polished.  Fire polishing is not possible as the drop would collapse long before the rim was smooth.

Because the rim will be relatively thin, it is possible to grind and polish with hand pads.  However, it is quicker to use a flat lap or linisher with a succession of finer grits to grind and polish an edge.  HIS Glassworks has a series of videos and this one gives good information on the methods and progression of grits to get to a polished edge whether by machine or by hand.

Aperture Drops Annealing

The soak at annealing temperature will need to long to accommodate the temperature variations within the thick and thin parts. The thin parts will be able to cool much faster than the thicker parts.

The objective in annealing is to keep all parts of the glass cooling within a 5C range, so the soak will need to accommodate those differences. I suggest a minimum soak time is 90 minutes for a 9mm thick blank, 2 hours for 12mm and 3 hours for a 15mm blank to be certain all the glass reaches the same temperature.

Annealing the drop has two main considerations – the variation in temperature over the length of the piece and the variation of thickness of the glass. These two in combination make it difficult to find a rapid annealing and cooling schedule. So having spent quite a bit of time so far on the piece, choosing a conservative schedule is sensible.

The variation in temperature between the top and bottom of the kiln can vary quite a bit, maybe as much as 20C for some kilns. So you can see immediately that the annealing will need to be slow if you are going to keep the thick and thin glass within 5C of each other. It would be possible to use schedules for annealing thick pieces just as they are published for the thickness of your blank, but it is more conservative to use a cooling schedule for the next size up to ensure a good anneal.

Thus, for a 9mm piece I would anneal at 55C/hr for the first 55C below annealing, then 99C/hr for then next 55C. After that you can go much faster. For a 12mm piece I would go at 25C/hr for the first 55C, 45C/hr for the next 55C and 150C/hr to room temperature. For a 15mm piece I would go at 15C/hr for the first 55C, 27C/hr for then next 55C, and 90/hr to room temperature.

It may be possible to go faster than this in annealing, but this is cautious to make sure the variations in both thickness and temperature are considered.

Aperture Drops - Stopping the Drop

Arresting the drop and cooling the piece can be complicated, as you need to cool the kiln quickly enough to stop the glass moving.

The higher the forming temperature of of the work, the quicker you need to stop the movement of the glass. This will involve opening the kiln to cool the glass enough so that it becomes stiff and resists further movement. You need to be aware that you are cooling glass that ranges in thickness from relatively thick to relatively thin. The thin parts will cool faster than the thick parts. Flash cooling for too long will make the thin parts very stiff, while the thicker part are still hot. This could lead to breakage if allowed to continue down to the annealing soak temperature.

A lower forming temperature will allow you to simply advance to the rapid cool portion of the schedule down to the annealing soak without the need for flash cooling. You do need to make this skip to the next segment just a minute or so before the piece reaches its desired length or shape. This will not be difficult to judge as you will have been checking frequently at this portion of the firing.

Aperture Drop Observation

This kind of firing absolutely requires observation of the progress of the drop. Ideally you would set up the firing surface where you can peek at it during the firing as well as observe the bottom of the kiln or the shelf – which ever you are firing upon.

I you have to choose, then the bottom of the kiln is the most important place to have clear observation lines. Even if you do not want the drop to touch the shelf or bottom of the kiln, you will need to observe how far the drop has progressed.

Thus, planing for the placing of the supports and other elements of the drop are important. Support posts should not obscure the view of the drop, for example. The whole set up should be placed far enough back in the kiln to see the shelf/kiln bottom where the glass will touch down.

If you do not want to have the drop touch down onto a surface, you need to set up a “witness” to indicate how far the glass has fallen. This can be some pieces of fibre stacked up so that your view through the peep hole to the top visible surface of the “witness” will tell you that when the glass touches that line of vision, it has reached the desired length.

You need to patient, as the soaks can be two or more hours long for a low temperature drop.

Aperture Drop Placement

Aperture drops normally are placed much higher in the kiln than most work to get the greatest length of drop. This means that the glass is near the elements and so will be heated unevenly. It has been said that the heat evens out across the kiln approximately the distance below the elements that they are apart. So if the elements are 100mm apart, the heat will even 100mm below the elements. This constraint means that it is difficult to get the length of drop wanted and still have the glass heat evenly.

There are at least two things you can do to get more height. One is to take out the shelf and its supports so you can fire on the bottom of the kiln. This will give up to 50mm extra drop length.

The other is to go ahead and fire closer to the elements than is indicated for even heating. This will require radical modification of the heating schedules. [qv firing]

Grinder Head Grub Screw

Need help! The small screw that secures the grinder bit to the shaft was stuck and my efforts to loosen it resulted in stripping it. I've tried spraying it with lubricant -- still no luck. What can I do?

There is a tool that many mechanics and tool shops have. It is normally square or triangular. It is used by drilling into the broken off bolt, or in this case, the grub screw. The tool is hammered into the hole and then with a wrench/spanner loosened.

However, you should make sure that the socket for the allan key is clear of glass residues. I do this by using a needle or other thin sharp object to clear out all the glass powder. I am sure there are other things to clean out the hole too. When the socket is cleaned, I push the key into the socket very firmly and hold it there while turning. This has worked for me in the past.

Once the grub screw is out, you need to get a replacement, so the same problem does not re-occur. I keep the screws from old heads in my box of grinder parts for this eventuality.

So the maintenance is not only on the shaft but also on the fixings. Putting a dab of Vaseline or thick grease into the socket will help keep it clear of the glass residue.

Aperture Drops Firings

Initial Heat Rise

As the placement of aperture drops is much higher in the kiln than normal, the initial heat rise needs to be carefully controlled. Usually, the glass will be so high in the kiln that uneven heating is almost certain and the risk of breakage very high. The need is to arrange a schedule that takes account of this uneven heating effect.

The principle requirement is to add heat slowly so the glass receiving less direct heat can heat up by convection through the glass. However glass is a very good insulator, allowing heat to travel only slowly. There are two strategies for this:

• one is to heat at a very slow but consistent rate. After the annealing point has been reached the speed can be increased.
• the second is to go a bit faster, but with soaks at three or more intervals in the heat up. After each soak the speed of advance can be increased a little. The soaks should be from 15 to 30 minutes, depending on the speed of heat up.

In either case it should take about five to six hours to reach 650C for 9mm thick glass. If the glass is thicker, more time is required to get to this point. I would take 8 -9 hours for 12mm glass; 16 – 18 hours for 15mm glass; 26 – 30 hours for 18mm glass.

Bubble Squeeze

If the glass has not already been fused, you may need a bubble squeeze at around 650C. Keep in mind that the temperature rise has been slow and so a lot of heat has been put into the glass. A quick peek can tell you whether the glass has already sealed at the edges. If the glass was per-fused, you can continue directly to the forming temperature.

Forming temperature

The exact forming temperature of course is dependent on:

• aperture size
• weight of glass
• speed of advance to forming temperature
• glass used (to a lesser extent)

However the forming temperature will be between a high temperature slump and a low temperature fire polish or tack fuse. Observation will be required to determine the temperature for your kiln.

Soak at forming temperature

It is best to soak for a long time at the forming temperature. At high temperatures the glass will move quickly, possibly too quickly to arrest the movement when you want. At higher temperatures the glass thins much more at the shoulder – where the glass moves from the horizontal to the vertical – than at lower temperatures.

Lower temperatures take longer to form, but are more controllable. More of the glass has time to slip into the aperture. Lower temperatures allow compensation for the increased speed of the drop during long drops. After the first 50-75mm of drop the glass at the sides is thin enough to allow a quicker drop caused by the weight of glass at the bottom pulling on the thinner sides.

Aperture Drop Supports

The supports for aperture drops need to be rigid at tack fusing temperatures. A number of materials are rigid enough to maintain their form. Those such as ceramic, or fibre board are commonly available. The ceramic forms can be purchased from various suppliers. Fibre board can be carved in a number of shapes and so are more versatile. They are more flexible than ceramic so need careful support.

The supports also need to be of such a material that will not trap the glass when cooling. This makes metals unsuitable for use as drop supports. The metal contracts more on cooling than the glass does, and so traps or crushes the dropped part of the glass.

Note that the supporting structure does not have to be flat. It could slope toward the centre, or could be curved down on the outside. The permutations are up to your imagination.

The other element of support is the material to hold the support surface above the kiln floor. These supports need to be stable so should have a relatively broad base in relation to the height of the support. Two good kinds of supports are kiln posts and fire brick sawn to the appropriate height. There other possibilities to create home made kiln furniture. [qv]

Note that it is important to kiln wash all the supporting materials to avoid any glass getting stuck to them.

Aperture Drops – Length of Drop

The height of the drop is related to the thickness of the glass.  The glass moving at the edge of the hole becomes thinner than the rim, so the deeper the drop, the thicker the glass required.

The general rule of thumb is to have 6mm for the first 50mm drop. For each additional 50mm an additional 3mm of glass is required. So, by this method a 20cm drop will require glass at least 15mm thick.

A more accurate method is described by Frank van den Ham in his book – Kilnforming Glass, a Master’s Approach.  This is based on obtaining an approximately 4mm thick rim and relies on measuring the amount of glass needed to provide an average wall thickness of 4mm.  The method is:

• Double the drop length, and add the diameter
• Divide the result by the diameter
• Multiply that result by 0.4cm (the average thickness to have a robust result)
• This gives the resulting thickness of glass required in centimetres.
• Divide centimetres by 2.54 to get the decimal part of an inch.

This method relates the diameter (or other dimensions of the opening) to the length of the drop. 

By this method a 20cm drop through a 20cm aperture would require a 1.2cm/0.5” thick blank.  If it were to be a 30cm drop, a 1.6cm/0.625” thick blank would be required, but by the rule of thumb, a 2.1cm/0.825” blank would be needed.

However, if you have a blank and want to know how far you can safely drop it you can determine it by:

• Thickness (in cm) divided by 0.4cm
• multiply by diameter
• subtract the diameter from that result
• divide this result by 2 
• This gives the length of the drop safely possible in cm.
• Divide centimetres by 2.54 to get the decimal part of an inch.

By this method a 12cm aperture with a 1.5cm (5 layer) blank would require division by 0.4cm to give 3.75.  Multiply that by 12cm (the diameter of the aperture), giving 45cm, subtract 12cm and divide the result by 2 which gives a thickness of 16cm or just over 6 inches.

The thinning effect of the stretching can be influenced by both the temperature and material of the supporting material, so this method cannot be infallible.

Revised 14.12.24