Cleaning Blending Brushes

Cleaning badger brushes just before use, is easy. Flick, gently and rapidly, the very ends of the brush hairs against the side of your hand – but use respiratory protection and be careful not to inhale any dust. If you notice flecks of dust in your paint when you create a grisaille you’ll know it’s time for a thorough and wet cleaning again.

After each use, rinse out the brush tips in cool water. Gently rub the tips of the brush hairs to loosen any extra paint. Grasp he hairs above the tips to keep the water from the main part of the brush. Then wet the exposed ends of the hairs and rub them gently until the water runs clear. 

If you use a blender for oil, you will need to use a small amount of natural soap, if so, thoroughly rinse.

Flick the brush to remove excess water, smooth the hairs into shape and allow to completely dry by hanging the brush with the hairs pointing downward – this avoids water flowing into the brush base where the hairs are attached. If you have round-handled brushes, you can twirl the brush between your hands to remove excess water.

Shape of Aperture Drops

The shape of an aperture drop can be controlled by the speed of the slump. The speed at which the glass drops is a combination of heat and size of the hole. Patience is required.

Rapid drops result from high temperatures. Rapid slumps cause a thinning of the glass at the shoulder where the glass turns over the inner rim of the aperture. The pattern is distorted and the colours are also diluted. And a relatively large rim is left around the fired piece.

A much slower rate of drop spreads the strain of the slump over the whole of the unsupported area of glass. This tends toward a bowl with a gentle slope toward the bottom, reduced distortion of the pattern, maintenance of the colour densities, and a more even wall thickness all over the piece.

The slumping temperature for a shallow angled slump is less than that used for normal slumps, and takes a lot longer – up to five hours typically. This means that observation is required at intervals, say every half hour.

A starting point for the slumping is around 100ºC above the annealing temperature for the glass. So for Bullseye and System 96 the temperature is about 615ºC. If after the first half hour, there is no movement, increase the temperature by 10ºC. Check again in another half hour and if the slump has begun, leave the temperature at that level and observe at the half hourly intervals until the desired slump is achieved. Otherwise, increase the temperature by another 10ºC with the check after half an hour, and repeat until the slump has begun. After you have done the first one of these with a particular size of aperture, you will know the temperature to start the slump.

The temperature you need to use is affected by the size of the hole. The smaller the aperture, the higher the temperature will be needed. But be patient. If the temperature is increased too much, you will get the thinning of the sides that you are trying to avoid.

Additional information on aperture drops can be found in this series.

Lead Came with Alloys

Lead came is available in several hardnesses. One (soft) is almost pure lead, another is half hard and contains up to 5% antimony, and the third is hard, containing up to 10% antimony. The difference between these is hardness, or resistance to creep, not resistance to corrosion.

elemental lead

Lead with antimony as an alloy is subject to the same corrosion rate in atmospheric environments as chemical lead (99.9% commercial-purity lead). However the greater hardness, strength and resistance to creep of antimonial lead often makes it more desirable for use in specific chemical and architectural applications.

The ability of some antimonal leads to retain this greater mechanical strength in atmospheric environments has been demonstrated in exposure tests in which sheets containing 4% Sb [antimony] and smaller amounts of arsenic and tin were placed in semi-restricted positions for 3 years. They showed less tendency to buckle than chemical lead, indicating that their greater resistance to creep had been retained.

Handbook of Corrosion Data, by Bruce D Craig, p89ff

Antimony crystals

Thus, the use of softer leads in conservation or restoration, because they were used in earlier periods, is not indicated. It is known that lead came up to sometime in the early 19th century was melted and re-formed into came, incorporating tin from solder and other trace elements which made the lead “stiffer” than the more pure lead that began to be produced commercially and used widely at that time. This may be the reason that so many 19^th century windows contain failing leads, while many earlier ones remain sound.

Pink Confetti

Because confetti needs to be so heavily saturated with colour, some of the opalescent colours tend to devitrify. The pink is particularly prone to devitrification. There are several ways to prevent this: 

• cap (which can lead to bubbles), 
• add a devitrification spray, or 
• cover with clear powder or frit.

Covering completely with a fine layer of powder gives the most even result. Using frit can provide a speckled appearance that is useful in some circumstances.

This tendency of pink opal to devitrification applies to all formulations – Bullseye, Uroboros, S96 and float.

What is Viscosity

What is Viscosity?

An example of differing viscosities

There are a variety of definitions, but these two capture the main elements.

Informally, viscosity is the quantity that describes a fluid's resistance to flow. Fluids resist the relative motion of immersed objects through them as well as to the motion of layers with differing velocities within them.  Source

Viscosity is a measure of a fluid's resistance to flow. It describes the internal friction of a moving fluid. A fluid with large viscosity resists motion because its molecular makeup gives it a lot of internal friction. A fluid with low viscosity flows easily because its molecular makeup results in very little friction when it is in motion.  Source

A demonstration of the resistance of different viscosities of oil to a weight moving through the liquid.

Almost all liquids are viscous fluids having viscidity. For example, when rotating a drum container filled with water on its vertical central axis, the water that was at rest in the beginning starts moving as it is dragged by the container’s inside wall and then whirls completely together with the container as if it were a single rigid body. This is caused by the force (resistance) generated in the direction of the flow (movement) on the surfaces of the water and the container’s inside wall. A fluid that generates this kind of force is regarded as having viscosity.

Temperature is a very important factor for measuring viscosity. In fluids, as temperature goes up, viscosity goes down and vice versa. In the case of distilled water, if the temperature changes 1 centigrade, it produces a difference of 2 % to 3 % in viscosity.  Source

Viscosity is the measurement of a fluid's internal resistance to flow. This is typically designated in units of centipoise or poise but can be expressed in other acceptable measurements as well. Source

Why is viscosity important?

“Near the strain point the expansion increases rapidly and sometimes erratically.” The links between the molecules has reduced in strength and so have a lesser role in the forces acting at higher temperatures. “In those upper ranges – the temperatures where glasses are formed and re-formed with heat – viscosity is a much more useful indicator of how glasses will behave.

“The combination of viscosity and COE are what make glasses more or less compatible, i.e., containing stress in amounts low enough to allow them to hold together without breaking at room temperature for extended periods of time under normal circumstances.

Bullseye found in the early 1980s in their efforts to mix coloured glasses in streaky colour combinations that the COE could not be used to predict compatibility. In trying to correct the compatibility of certain mixed glasses, the closer they brought together the COEs, the more incompatible became the mixes.

“The reason that we could not use COE to successfully predict whether a coloured glass would fit the base clear glass was/is because, as the base glass composition is altered with the addition of the necessary oxides to colour it, the viscosity is inevitably changed. This viscosity change causes the coloured glass and the clear base glass to strain themselves in the cooling cycle of the fusing process (a viscosity mismatch). Therefore once the two glasses reach room temperature they have undue residual strain that may lead to failure.

“In order to prevent this undue residual strain an equal but opposite strain must be introduced into the coloured glass to cancel out the strain induced by the viscosity mismatch. This is accomplished by introducing an expansion mismatch of equal but opposite strain. The two mismatches cancel each other out, leaving the two glasses nearly strain free.

“It is this phenomenon (viscosity mismatch cancelled out by an equal but opposite expansion mismatch) that enables glasses of very different compositions to be formulated to fit each other. The very fact that the expansion of a coloured glass has to be altered to make it fit a base clear glass implies that COE cannot be used as an indicator of compatibility. It is also why it only makes sense to describe these glasses as tested compatible to a specific manufacturer's base glass for a specific glass forming process.“ [L. MacGreggor]

Even different formulations of glass have different viscosities and different rates of softening with temperature increases.

How does viscosity apply to us?

Although viscosity is of major importance to the manufacturer, it does have some relevance to kiln formers too.

Understanding that glasses have different viscosities – most often referred to as hard and soft – can help in the choice of colours and styles of glass to combine. Some glass will spread more, and also allow other glass to sink deeper into the layer than others. It might help avoid combining extremely hard and soft glasses next to each other.

It should also help explain some results that were not planned. It may help in when thinking about uneven slumps.

It is important to recognise that glass chemistry is extremely complicated, and to see that the expansion characteristics have to be balanced with the viscosity characteristics as the two main elements in compatibility. There are others, of course, but these appear to the two main ones.

Bubble Reduction in Casting

There are several things that can be done to reduce the number and size of bubbles in casting.

• Fire higher - to 830ºC instead of 815ºC - and soak for at least four hours. This allows more bubbles to rise to the top and burst. If there are still more bubbles than wanted, increase the soak time.

• Stack the glass in the centre of the mould, allowing a few centimetres from the mould walls. This allows the glass to spread and flow from the bottom and up the sides, reducing the likelihood of trapping air. If you have more than one stack, keep the same space between the stacks as the mould walls.

• Make sure that the way you stack the billets or casting plates so there is a smaller space at the bottom of any cavity than at the top. The reverse allows the glass to soften and seal in the air in the space.

• You can construct a mould to make billets of the general shape of the final object. This of course, is much more work, needing two moulds.

• A major thing to avoid is the use of frit, especially at the bottom or deep in the mould as bubbles will collect around each piece and lead to a multiplicity of bubbles throughout the casting.

Thinfire as a Separator on Moulds

"I was told that it was possible to put a piece of thinfire paper under a circular piece of glass that is to be draped over a mould....  Has anyone done this or heard of doing it?"

Yes this can be done. Some caveats are in order though.

It is important to put a separator on the mould -normally kiln wash - before using it. Once coated, it will not need to be re-coated unless the surface is damaged, scratched, etc., for a very long time. If the kiln wash seems to be rough you can smooth it in various ways as noted in this tip.  The advantage of kiln wash is that it does not cost much and lasts a long time. The thinfire or similar is a one-time-use product.

The binder in the thnifire burns away during the heat up and allows the now unbound separator to drift down to the mould before the glass begins to conform to the mould. In that way it is very similar to a fine dusting of kiln wash powder over the mould surface before firing.

I don't see the point of using Thinfire or similar during slumps (although I can see that there are occasions where this method would be useful). For drapes, placing a bit of thinfire over the mould - especially if it is metal – is an additional precaution.

My view is that you have to kiln wash the mould anyway. KIln wash is cheap and long lasting while thinfire needs to be replaced after each firing.

Clearing Small Core Drill Bits

Core drill bits are very useful, as only a thin wall of glass is removed, leaving the main part of the hole as a single piece. This means that it is quicker than a solid drill bit through not having to remove so much glass. However, small diameter core drills often jam up with waste glass. This is evident when the drill ceases to drill through the glass, leaving a broad circular mark as it moves around on the glass.

Inevitably, at one time or another, the glass core will get stuck inside the drill bit. This needs to be cleared before any further drilling can be successfully done. It seems to be more common with the smaller diameter drills – from 5mm down.

The broken off core needs to be cleared from the back of the bit, not the diamond coated end. So you need to take the bit out of the drill and use a thin nail or piece of wire into the hole to push the glass out.

My collection of core drills.  At the front you can see copper wire and a thin punch for clearing blocked drills

Core drills need a more accurate means of steadying the bit than your hands. Imagine that you are trying to get that core out of the glass in tact. Any wobbling as you drill will break that core. When the core breaks, it is inclined to jam the cylinder. So a drill press is almost essential when using a core drill.

They also need water like any other drilling of glass. Normally this is supplied through the hollow core of the bit, which is beyond hand held drill motors.

For something as smaller than 3mm, a solid drill that removes all the glass rather than a core is more likely to be most successful. You can get diamonds bonded to piano wire that will give you holes from a fraction of a mm up to 2 or 3 mm and these can be used in dremel-like tools that take small diameter bits.

It is absolutely NOT recommended to use a core drill at an acute angle to start the hole. The wall of a 3mm core drill is very thin and easily damaged. It is ok to start a solid drill that way, but not a core drill.

There are a number of related posts beginning with this one which give advice on the many ways to drill holes in glass.

Finger protection

Grinding lots of glass pieces often leads to a number of small cuts on the tips of your fingers. There are several things that can be done to reduce these cuts and the tenderness that comes from lots of grinding.

The first thing is to take the sharp edges off the glass. You can do a simple, light grind all the way around the piece. This removes the extra sharp edges that often remain after breaking the glass.

You can go a step further and do a light arris around the piece. This is just lightly holding the glass at about 45degrees to the grinding bit and going all the way around on all sides. This does not take off the shape of the piece, but gives a more rounded feel to the piece.

Illustration of the effect of holding the glass at an angle to the grinding head - not so much needs to be taken away as in the illustration to get the effect

It is not generally recommended that you wear gloves around rotating machinery. There is too much risk of injury, even on a small grinding machine. Some of the alternatives to gloves include plasters (band aids), masking tape, electrical tape.

Other purpose-made things you can buy include rubber finger protectors, finger tip pads, finger caps (as used in counting money).

Other tools are made to hold the glass such as the grinder cookie

and Nick's Grinder's Mate 

Reclaiming Solder

Re-using solder can range from simply soldering the ends of the solder sticks together (if you are using blowpipe solder). This will then form a useable stick with solder blobs on its length.

If you have a number of blobs and splashes, don't throw them out. Collect them together and when you have enough you can make them into another stick of solder.

You can tape two narrow pieces of glass onto a length of marble or heavy steel about 3-4 mm apart. Put the pieces in the channel formed by these two pieces of glass and melt the pieces with your soldering iron. This will form a useable stick.

Lifting the new solder stick from the wood

The same can be done by cutting out a 3mm wide and deep channel in a piece of timber and doing the same as above. The wood will smoke a bit and blacken, but not ignite.  And you can use a blowtorch to melt the solder if you do it this way.

Using the (slightly irregular) solder stick

Assembling Foiled Pieces

Keeping foiled pieces together while assembling them prior to soldering is sometimes a problem.

If the panel is rectangular - or at least one with multiple straight sides - you can use short battens of the length of the sides. Nail or screw them down to a board so that about one half of the panel is contained. So if it is a rectangle, two sides will be enough. If it is six-sided ,three pieces would do.

An assembled piece illustrating the two battens - although with a leaded panel

For ease of assembly, a copy of the cartoon should be fastened to the board first and then the battens fixed on top of the cartoon. The pieces can then be placed against battens and held there with pins or nails until the next pieces are ready.

Illustration of the cartoon fixed by the surrounding battens

Some prefer to tack solder the pieces together as they foil. This can be done in combination with the use of battens. However, leaving the soldering iron idling while foiling and using it only occasionally is very hard on the iron's tip and your electricity bill. I prefer to assemble the whole and then solder all at once.

Illustration of placing pins all the way around an oval panel

For circular or irregular shapes a slightly different approach is required. You can use multiple pins or nails along the perimeter to hold the foiled pieces together. An alternative is to cut a piece of scrap window glass to the shape of the external perimeter of the panel. Hold it is place with nails or pins and proceed as with a rectangular shape.

Placing nails all the way around an irregularly shaped piece