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

Aperture Drops Introduction

Aperture drops are apparently simple to do. But to have control of the process and to be able to get repeatable results is relatively complex. There are various elements that need to be considered when preparing to make one of these. The main technical considerations are:

The height of the drop from the shelf.

Material of the supporting ring or material.

Diameter of opening of the aperture.

Size of the blank in relation to the aperture

Initial firing speeds

Height in kiln and relation to the distance from the heating elements.

Observation of the progress of the drop.

Arresting the drop

Annealing and cooling.

Finishing the resulting drop.

The above instalments will discuss these in turn.

Scoring Opalescent Glass

Cutting opalescent glass often gives difficulties in getting clean breaks along the score line. You need to remember that the opals do not make much if any sound when cut with the correct pressure. If you are scoring so that you hear the ziiip sound, you probably are pressing too hard. When the score is too hard, the opals do not break easily or truly. Only the same pressure as used on transparents is required. Feel the pressure rather than listen for the sound.

Care in the Operation of Soldering Irons

The most important element in the deterioration of soldering iron bits is long idle times. This is where you leave the iron on, and not in use, for a long time.

Have everything ready when you start soldering, so the iron will be used continuously, and will not sit there building up heat, while you get ready to use it again. An idle iron will keep heating to its maximum capacity, and without anything to transfer the heat to, it will start burning off the tinning, after a short while. So if you will not be using the iron for a while turn it off until you are ready again.

The other elements leading to deterioration in performance come from lack of cleaning and tinning of the tip. When the coating of solder burns off or is coated with carbon you get poor heat transfer from tip to working surface making it appear that the iron is not heating properly.