Ramp Speeds for Slumps and Drapes

I find that a steady slow heat up gives best results for both slumping and draping. Many do a two stage heat up with a slower initial rise to a temparature above the annealing point and then faster afterwards.

A slow steady increase in temperature allows all the glass to be nearly the same temperature both across the piece and from top to bottom.

Speed in draping, especially where there are different colours and thickness, promotes "wings" as any unevenness in the heat across the piece will translate into variable drape speeds. Allow all the glass to get to the same heat, by going slowly.

I proceed at the same speed from start to the forming temperature - no speed up at all. Yes, the glass will accept it a rapid heating to the target temperature, but it does not promote even slumps or drapes.

Candle Bridge Moulds

A candle bridge mould is one of the most difficult moulds to use successfully. The problems relate to the kinds of work you are trying to do, the size of the glass to put onto the mould, the shape of the piece and the stability of the resulting piece.

With a candle bridge you are trying to do a combination of slumping and draping at the same time. You are slumping into the middle and draping over the curved sides.

Additionally the candle mould requires the glass to fall into a small opening and this requires long soak times. Long soak times mean the glass that is draping stretches while the central portion is trying to fall into the opening. Of course, if you don't want the depression to be flat, you don't have to soak so long and the stretching effects on the draping part of the of the glass won't be so great.

It would seem logical to measure the mould around the drapery curve (or arc of the mould) and to the shape of the ends, but experience has shown me that this leads to glass that is too long along the sides and bent at the ends. So I cut my blanks for candle moulds as a rectangle without curved ends and then round the corners of the rectangle just a little by nipping them with my grozing pliers before fusing.

If you measure along the top and along the length of the mould you have a piece of glass that will be increasing in length at the draping part of the mould, meaning that it will fall off the curve and onto the draft (or side) of the mould. The draft is an angle from the vertical. Good moulds are made with a draft so that if glass were to fall over the edge it still will be possible to get the glass off the mould.

The draft on a mould means the diameter of a circular one is greater at the base than it is at the rim. And it is common to measure only the diameter at the rim.  In the same way the dimensions at the outside base of a rectangular mould are larger than the rim of the mould.

Back to the rectangular candle mould. The draft on this means that measuring the base of the mould is slightly wider than the curved part of the mould, but less than if measured around its curved portion.

Experience has shown that in the case of the candle moulds measuring the width of the mould is sufficient. There is enough height in the moulds I have used that it does not make any functional difference if the glass does not reach the bottom of the curve on the mould. It is better than hanging off the edge.

The length of the glass should be no longer than the shortest part of the mould's length. Cutting a curve into the glass to allow a small overhang produces a depressed lip because of the length of the soak required for the slump into the small aperture of the candle depressions.

My soak for candle moulds is 90 minutes at my process temperature. This gives me a flat depressed area for the candle to sit, but it also means that the draping glass has been stretching. And it also means that the glass will drape unevenly as the various colours absorb heat differently allowing some parts of the glass to stretch more than others.

The placing of the glass on the mould is absolutely critical. It must be exactly parallel to the sides of the mould. Any slight movement from that will induce a twist in the resulting piece allowing it to rock. Arranging it exactly right and placing some kiln washed furniture at each side to keep it in place until it begins to slump is an important aid.  
The glass will begin to bend before it sticks to the kiln furniture.

I have never been able to get a stable candle mould whether from 3mm or 6mm thick glass. I always have to grind the base a little to make a stable piece. I take it as part of the process, but careful placing reduces the work.

Cutting Box Hinges

Cutting the brass tubing for box hinges is best done with a saw.

The manual version is to use a fine toothed saw - similar to those used by jewelers – with very light pressure.

The power solution is to use a Dremel or similar hobby motor with a cutting wheel attached. This cuts through the tube cleanly and quickly.

In both cases, a fine rat's tail file can clean up any burrs within the tubing.

If you use the internal tubing you can cut both at the same time. However, it is quick and easy to use the appropriate sized copper wire to insert and then bend down onto the corner of the box. This gives greater flexibility and avoids waste.

Vase Cap Fitting

“Can anyone offer me any tips for fitting the brass caps onto lampshades?””

There are at least three ways to get the right vase cap size.

Make up your shade in a cardboard mock-up. Use 3mm thick card or foam board to represent the glass, as the thickness of the glass is important in determining which vase cap is the correct size. Try your vase cap against the cardboard model, then if you need, alter the pattern so the glass pieces meet at just the right place to make the lip of the vase cap fit just over the top of the glass. You can do this by either shortening or lengthening the pattern a little at the top edge.

The second also involves making a cardboard mock up. After making this maquette, choose a vase cap that overlaps the top opening, covering all the edges. The third option is to use two vase caps, one above and one below the opening to clamp them together trapping the edges of glass between them. Use a furling and lock nuts with no solder at all to hold the lampshade together.

A general discussion of panel lamp dimensions

Squaring Panels

When building leaded glass panels to be placed in existing wooden frames, you need to make sure the panel is as square as the opening in the frame. The first requirement is to make sure your cartoon is squared, or has right angles at each corner.

I use a “roofing iron” as it is called in the UK. It is a steel tool about 600mm on one side and 400mm on the other. Its original use was to work out the pitch of roofs and check the same pitch was maintained all along the building. The first important thing – now that all roof trusses seem to be prefabricated – is that they still are in production. The second thing is that they all have a fixed right angle.

Using this roofing iron will ensure your corners on the cartoon are right angles. This helps in the drawing of the cartoon as you only need set the iron on the base line and draw the verticals without having to measure the width higher up the cartoon. Of course you should check that the width is still correct at the top, just in case there has been a slip.

Then you have to stick to the cartoon.

When you are setting the battens to ensure the sides are held where you want them while you continue with the leading, the roofing iron again will ensure that you have placed the battens at right angles. You choose which line is to be your base, and nail or screw it into place. Ensure it is exactly parallel to your cut line and then align one side of the iron against it. Place the other batten snugly along the length of the other leg of the iron and you know you have a right angle.

When you are completed leading, but before soldering you can check on the accuracy of the angles by using the roofing iron again at each corner to check on the “squareness” of the whole panel. If the panel is out of square, you can tap on the battens not yet nailed/screwed in place to ease it all back into “square”

Vase Caps

Attaching the vase cap securely is important as often the whole lampshade hangs from the attachment points between the cap and the solder seams of the shade.

Once you have assembled the shade and tack soldered it together, perch the vase cap on the top covering the opening and apply solder so it joins the vase cap with the solder seams. It is a good practice to turn the lampshade over and apply solder from the seam to the inside of the vase cap. A good strong joint at each seam will be perfectly strong enough to hold the shade in position for many years.

Another another method of attaching vase caps will be given soon.

Tinning vase caps

Sieves, Gauges and Grits

The commonly used designation for grits has become the gauge This is a confusing measure as it increases in number as the size of the material decreases in size. This is because the number of wires per unit increases with decreasing size and the gauge refers to the number of wires used to sieve the material.

In an attempt to indicate the actual sizes of material refered to by the gauge sizes, I have used part of a standard table of equivalents.

12 gauge is 1.7mm or .0661inch

14 gauge is 1.4mm or .0555inch

16 gauge is 1.18mm or .0469inch

18 gauge is 1mm or .0394inch

20 gauge is .85mm or .0331inch

25 gauge is .71mm or .0278inch

30 gauge is .6mm or .0234inch

35 gauge is .5mm or .0197inch

40 gauge is .425mm or .0165inch

45 gauge is .355mm or .0139inch

50 gauge is .3mm or .0117inch

60 gauge is .25mm or .0098inch

70 gauge is .212mm or .0083inch

80 gauge is .18mm or .007inch

100 gauge is .15mm or .0059inch

120 gauge is .125mm or .0049inch

140 gauge is .106mm or .0041inch

170 gauge is .09mm or .0035inch

200 gauge is .075mm or .00295inch

230 gauge is .063mm or .0025inch

270 gauge is .053mm or .0021inch

325 gauge is .045mm or .0017inch

400 gauge is .038mm or .0015inch

450 gauge is .032mm or .0012inch

500 gauge is .025mm or .001inch

635 gauge is .02mm or .0008inch

Soldering techniques

My experience leads me to say that the tip of the iron should be in contact with the surface of the material being soldered. If the metal is not hot, it will not take the solder well. In the case of copper foil, the metal is so thin it will heat up almost instantaneously. The solder should be added to the heated metal to obtain a good joint. All the advice to hover just above the surface and allow the molten solder to heat the metal below seems to make for hard work suspending the iron, and for possible cold joints.

The principle is that both metals should be hot for a good join. In leaded work you can sweat a joint and get as good (and in some way a more lasting) joint as by having a bead. That is because by adding the minimum of solder (sweating) you will have to get the base metal hot for the thin layer of solder to flow. I feel that many people do not understand the principles of soldering, but look only to the finish. It is possible to have a beautiful joint, or bead and have the joining of the metals technically weak.

Residues

Sometimes a white residue appears alongside the solder bead sometime after a piece is finished, covered in patina, and waxed. This seems to have two causes.

First - Residual acid

As there is a possibility of some acid remaining on the piece, rinse with a bit of bicarbonate of soda added to the water. This will neutralise the acids from the flux and patina that may still be lurking under the foil and solder beads. If you get a frothing while cleaning with the soda, you will know there is still acid present. Wash - rather than scrub - those areas again with the water and soda solution until there is no foaming.

Second - Trapped moisture

Moisture can also produce this as it allows minerals to migrate from under the solder seams. The advice seems to centre on cleaning. First do not use much soap in the initial cleaning solution. After rinsing ensure the piece is completely dry by setting it aside in a warm dry place for a day or two before waxing or sealing.

Those that use paste waxes seem to have less difficulty than those who use thin or spray on waxes. The heavier waxes seem to seal any moisture within the structure. The moisture seems to be able to migrate through the thinner waxes. It is not optimum to have moisture sealed within the panel, as it will eventually come through the wax as it ages.

So it seems the best long term result will be achieved by ensuring everything is absolutely acid free and completely dry before waxing.

Scoring Glass

Cutting glass is done by “scoring” the surface of the glass with a glass cutter, then breaking it along the score line. The break you make will always follow the path of least resistance, so you want to be sure that the score you make becomes that easy path and glass breaks the way you want it to.

Holding the Cutter
Generally, you use the cutter by moving it away from you, so you can see the cartoon lines as you score. When using a straight edge such as a cork-backed ruler to guide your cutter, you can pull the cutter toward you, or push it away as suits you. The cutter should always be held at a 90 degree angle (left to right). You can determine this by looking down the cutter to the wheel and to the cartoon line below.

It is important that the work be done from the forearm rather than the fingers or the wrist. The forearm should be held closely to the body. This reduces the freedom of movement, giving clean flowing score lines. It also reduces the actions that can lead to repetitive stress injuries. Any turning required by tight curves can be done by turning the body from the hips or shuffling around the bench with the glass at a corner.  Of course, for long cuts your arm will have to extend from you body in a parallel direction with the score line.

Scoring Pressure
The second and very important element in scoring glass is the amount of pressure used.  Very little pressure is required.  You should hear no more than a quiet hiss on transparent glass and almost no sound on opalescent glass.  However some manufacturer's transparent glass has almost no sound either.  So the important element is the pressure, not the sound.   Most people start with applying far too much pressure. Tests have shown that only about 2 kg of pressure is required for a clean score.

You can test the effect of this amount of pressure on a bathroom scale.  Place a piece of clear glass on the scale and without touching the glass with your other hand, score it noticing how much weight is being recorded.  Keep trying until you are at the 2 kg area of pressure.  Try breaking the glass.  Score a curve with the original amount of pressure and break the glass.  Then using the same curve score the glass with the 2 kg pressure and break the glass.  You will see and feel the lesser pressure provides a clean break.

Excessive pressure leads to breaks showing significant stress marks on the edge of the glass.  Too little pressure has no effect on the glass, making it impossible to break along the score line.  The correct pressure (ca. 2 kg.) leads to almost vertical stresses being put into the glass which assists the breaking along the score line.  Too heavy pressure creates stress marks which are at increasingly large angles with the increasing pressure.  This will still break cleanly on straight lines, but when working around curves the glass can follow one of the lateral stress marks away from the score line.  Excessive pressure is often the cause of glass breaking away from the score line on a curve, especially a tight one.

Foiling Nuggets

Grinding of the edges of the nuggets is not required for foiling. Roughing up the surface helps some adhesives hold better, but it depends on the viscosity of the adhesive and the degree of "roughness" of the surface. The adhesive on copper foil sticks better to a smooth than a rough surface. Try sticking it both to glass and to fine sandpaper and see which is easier to scrape off, for example. You will find the foil easily comes off the sandpaper, which is the texture of the surface you leave when grinding.

So you do not need to grind. You may need to wash them with soap and water to remove any oil that may be on the surface to ensure a good contact, however. Just put the foil on the clean nuggets. Then put a bunch of the foiled nuggets in a plastic container and shake around until all the nuggets are nicely burnished.

Check each one to ensure they are fully burnished to the nugget. Smooth any lifted parts of the foil with a fid and they are ready for soldering.

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

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.

Grinder Bits

Extending the life of your grinder bits is a matter of recognising that you should not force the glass into the grinding head. Excessive pressure against the head heats the bit and allows the diamonds to become free of the binding material, so reducing its life. If the motor slows as you press the glass to the bit, you are applying too much pressure. That kind of pressure also puts a lot of wear on the bearings of the motor.

If the grinder is not taking glass off fast enough for your purposes, you should put a coarser bit on the grinder, rather than pressing harder. The bits do come in a variety of grits. Try out some different grits to find the one that works best for the speed at which you want to remove the glass.

You can also buy a additive for the water – often called a diamond coolant – which is intended to provide a kind of lubrication for the diamonds. This may also extend the life of the bit.