Annealing Range

Among the critical temperature ranges is the temperature around the annealing point. These are known as the strain points. The higher one is the highest temperature that annealing can begin and is the softening point. The lower one is the lowest point at which annealing can be done and is called the strain point. Soaking at any lower temperature will not anneal the glass at all. The ideal point to anneal is the annealing point, because annealing occurs most quickly at this temperature.  This temperature is defined by various characteristics both mathematical and observational. The manufacturer can give this temperature, and most do on their websites.

This annealing range is traditionally calculated as being 40ºC either side of the stated annealing temperature. Your aim should be to spend as little time in the temperatures above the annealing range to reduce the chances of devitrification.

Most glass kilns are not really accurate in recording the temperature within the glass. They are measuring the air temperature. The glass on the way down in temperature is hotter than the recorded temperature. If you do a soak at 515°C for example, the glass is actually hotter and is cooling to the 515° point during the soak. So, long soaks at the annealing temperature are required. Longer for thicker is required. The slow cool to at least 5C below the lower strain point does the annealing, and reduces the risk of inadequate annealing.

Recent research at Bullseye indicates that the use of the fact that temperature readings are above the actual temperature of the glass indicates lower annealing points for thick glass. This has been based on temperature probes at various points within the glass, comparing the glass temperature with the air temperature. The results of their research suggest an annealing soak at about 30C below the annealing point with a long soak, and slow anneal cool for the next 55C.

It is still possible to give the glass a thermal shock at temperatures below the lower strain point, so care needs to be taken in the continued cooling. But no further annealing will take place. If you do not anneal properly, the glass will break either in the kiln or later no matter how carefully you cool the glass after annealing.

Maintaining a Single Colour on the Edge

The piece in the middle distance shows the different colours of the two layers

Keeping the edge one colour on a two or multiple colour piece can be done by cutting the upper layer larger than the lower one(s). If you are making a 6mm thick piece, the upper piece needs to be 3mm larger all around. So if you have a 300mm diameter base piece, the top will need to be 306mm in diameter. This allows a coloured top to bend over a clear base, giving the appearance of a single colour throughout.

However if you are building thicker than 6mm and are willing to allow the glass to flow, you do not need to add the full thickness of the glass to the size of the capping piece. I find that at 9mm thick, I need only 4mm all around to cover the layers. This may be because the outer edges are nearer 6mm than 9mm thick.

Of course, if the final piece needs to be a pre-determined size the lower layers can be cut 3mm smaller than the top all around (for 6mm thick pieces).  The top is cut to the final size of the piece.

Glass Dust

This is from Greg Rawls' website.  He is a glass worker and a certified industrial hygienist. A huge amount of practical information on safety in glass working is available on his web site:

http://www.gregorieglass.com/Health_Saf ... mical.html

Ground Glass

OSHA classifies glass dust as a “Nuisance Dust”. Ground glass does not cause silicosis. You can wear a respirator if you are concerned about exposure.

Glass is made from sand, which contains silica - a naturally occurring mineral silicon dioxide (SiO2). Crystalline forms of silica, also known as “free” silica, can contribute to the development of silicosis under prolonged exposure conditions.

It is important to understand the difference between glass and crystalline silica because exposure outcomes are extremely different! Glass is a silicate containing various other ingredients which have been melted and upon cooling form an amorphous, or non-crystalline structure. While silica (SiO2) is a primary ingredient in the manufacturing of glass, when glass is formed under heat, the crystalline structure is changed to an amorphous structure and is no longer considered crystalline.

Ground glass is rarely respirable because the particle is too big. Always use wet methods when grinding glass! Water captures the dust. Sometime other chemicals are used to add colour to glass such as arsenic, lead, cadmium. These are usually present in low concentrations and are bound to the glass and not readily available but could present an exposure issue under some circumstances.

Observation Ports for Kilns

Observation Ports for Kilns

When choosing a kiln, an often overlooked element is the observation ports. These openings in the side or top of the kiln enable you to observe the progress of your work during a firing without opening the kiln lid or door. They have ceramic or fibre plugs to keep the heat in the kiln when you are not using them to observe what is happening. 

A kiln with a very large quartz observation panel

Some newer kilns are built with quartz observation panels in the kiln. These serve the same purpose as the ports, but without the (small) additional heat loss.

When doing any new work it is important to observe the progress of work, rather than just hope for the best and see what has happened after the whole process is finished. Observation can tell you when the piece has reached the desired stage and progress to the next part of the programme.

A port located too high to be of use for observation of the interior.  It is sealed with a ceramic fibre plug.

The location of the port is important. You need to be able to see the relevant part of the kiln or they are useless. 

This relatively large kiln has two ports, one at the center of the door, and one on top.  The top is mostly for ventilation.  The one in the door may be too high to observe work while firing unless the shelf is put up on tall kiln furniture

Although a small kiln, the observation port at the top is not so useful as one at the side.

A popular kiln with an appropriately placed observation port.  Often these have an additional one on the side opposite the controller. 

 Some kilns have multiple ports to make observation of various parts of the kiln easier.

There are a variety of shapes of these ports. The shape is not so important as the location and what can be viewed within the kiln through the ports.

A round port, but probably too low to be of much use

A rectangular port viewed from the inside showing the field of view that can be allowed

A kiln with multiple square ports

If your kiln has come without a port or one that is not placed where most suitable for your use, you can drill the casing and brick or fibre to provide another viewing port.  Make a ceramic plug or wad up ceramic fibre blanket to fill the hole when it is not in use.

Boiled Glass

This is a technique that will obtain a random, organic feel to glass that would otherwise be scrap (cullet) – remembering that you have to use compatible glass throughout. The principle is to take the temperature up high enough for the glass to begin to flow easily and bubbles to blow through and burst.

The results can be used as they come out, or they can be cut to provide points of interest in other work, or the glass pieces can be damed before firing to obtain thick pieces which can be cut into slices for other work. And I am sure, there are numerous other ways to use the resulting glass too.

The effects are rather like colourful molten rock with gases bubbling through. These bubbles mix the glass colours. So you need to be sure you do not use a wide variety of colours, or your result will be similar to the molten rock - muddy. Use a few contrasting colours, and ensure you include a significant proportion of white to maintain bright colours. Also remember that the hot colours – reds, yellows, oranges – opalise at high temperatures, so the transparents can be used as opals.

You can use whole sheets of glass or scraps. In either case, it is useful to start with a clear base to help avoid picking up kiln wash when the glass is moving about. The glass must be clean to reduce the incidence of devitrification. Stack you glass on top of the base glass in what ever order you like. Contrasting colours alternated give a strong result.

You can put shelf paper of 0.5 mm or thicker on the shelf or simply kiln wash the shelf with several layers of wash until the shelf surface is no longer visible through the wash. Use of thinfire is not recommended as the powder can be pulled into the glass.

If you do not dam the area to contain the glass calculate how far the glass will expand on the shelf, so that you do not put down too much glass and have it spill over the edge of the shelf.  

You can use bubble powder onto the base layer to promote the bubbling during the firing. However, if you are using cullet, you can just take the temperature up rapidly without a bubble squeeze, which will give you plenty of air pockets to burst through the layers of glass.

You can take the temperature up at about 300ºC per hour to 925ºC with no bubble squeeze and soak for 10 – 15 minutes. Then allow the kiln to drop the temperature as fast as possible to about 815 and soak there for around 30 minutes to allow the little bubbles to rise to the surface an burst too. Then reduce to the annealing temperature and soak for the thickness you calculated in preparation for the firing.

Precautions

You need to be careful in firing and annealing pieces using this glass. Any glass that has been fired to a high temperature tends to begin changing compatibility. So you need to be careful on your rates of advance, and on the annealing and cooling portions of the firing when using the glass in other projects. You may want to consider using a schedule for twice the thickness of the piece on subsequent firings.

There may be devitrification on the surface. You should sandblast or abrade away this devitrification in some way to be able to get a shining surface when you fire polish.

There may also be a number of pin hole sized bubbles at or just below the surface. These will close with a fire polish also.

Lead Corrosion in Acids

Lead forms a protective film, which if undisturbed preserves the metal below this layer.

The corrosion resistance of lead is based on its ability to readily form a tenacious coating of a reaction product. This then becomes a protective coating. Protective coatings on lead may form as the result of exposure to sulphates, oxides, carbonates, chromates, or chemical complexes.

Handbook of Corrosion Data, by Bruce D Craig, p26

Lead is resistant to corrosion especially “with solutions containing sulphate ions, such as sulphuric acid.”

However, the new or bright metal reacts quickly with a variety of alkalis and many organic (although not most inorganic) acids.  “...Lead is not stable in nitric and acetic acids, nor in alkalis. The metal does not resist nitric acid. Lead corrodes rapidly in acetic and formic acids.” (Handbook of Corrosion Data, by Bruce D Craig, p.29)

Lead has very limited resistance to acetic acid.... Dilute [acetic acid], even at room temperature attacks lead at rates exceeding 1.3mm/year. These rates increase rapidly with increasing aeration and velocity However … acetic acid … has little effect at strengths of 52% to 70%.

The corrosion rate in acid increases rapidly in the presence of oxygen and also in oxygen in combination with soft waters such as rain and distilled water. Corrosion increases at the rate approximately proportional to the oxygen content of the water.”

Handbook of Corrosion Data, by Bruce D Craig, p.26, 29

This another good reason to avoid vinegar as a cleaning agent for leaded windows.

Lead dissolves in organic acids (in the presence of oxygen). Lead also dissolves in quite concentrated alkalis (≥10%) because of the characteristic of the lead salts that can act as either an acid or an alkali. These salts are soluble in the presence of water and oxygen.

Alkali salts are soluble hydroxides of alkali metals and alkali earth metals, of which common examples are:

• Sodium hydroxide (often called "caustic soda")
• Potassium hydroxide (commonly called "caustic potash")
• lye (generic term, for either of the previous two, or even for a mixture)
• Calcium hydroxide (saturated solution known as "limewater")
• Magnesium hydroxide is an example of an atypical alkali since it has low solubility in water (although the dissolved portion is considered a strong base due to complete dissociation of its ions).

Although this has been a rather technical posting, these data show that lead is subject to rapid attack by both organic (and some inorganic) acids and alkalis in relatively low concentrations when in the presence of aerated water. However in normal environmental conditions the protective reaction layer avoids much of this vulnerability.

Pot Melts – Weight of Glass Required

Circular pieces
This table assumes that a 150 mm diameter pot is being used, and assumes that 125 grams of glass will be left in the pot. Larger diameter pots or even pot trays can be used, but more glass will remain in the container. The following table gives the desired diameter of the melt and the weight of glass needed to achieve an average 6 mm thick result. To achieve a uniform six millimetre thick disk will require long soaks at both melting and fusing temperatures to allow the glass to even out in thickness.

50 mm diameter disk requires 154 grams of glass
100 mm diameter disk requires 243 grams of glass
150 mm diameter disk requires 390 grams of glass
200 mm diameter disk requires 596 grams of glass
250 mm diameter disk requires 861 grams of glass
300 mm diameter disk requires 1185 grams of glass
350 mm diameter disk requires 1568 grams of glass
400 mm diameter disk requires 2015 grams of glass

Thicker melts
Of course if you want a thicker pot melt — one that is confined so that it cannot grow larger, only thicker — you can use the following method to estimate the amount of glass required.

Choose the diameter wanted from the above table, and subtract 125 from the weight of glass required. Then multiply by thickness wanted divided by 6 mm. Add back 125 gms — the amount that will be retained in the pot — and you have the required amount.

For example: a 200 mm disk of 6 mm requires 596 gms. You want a 12 mm thick disk of 200 mm.
First subtract 125 from 596 to get 471 gms. 417 gms times 12 equals 5652. Divide this by 6 mm and you have 942 gms required. Add 125 gms — the amount left in the pot — and you have a requirement of 1067 gms for a 12 mm thick disk of 200 mm.


Rectangular pieces
These are easier to calculate than discs, as the calculation is length times height times depth (all measurements in centimetres).  

If you are making a billet and using an empty margarine pot of 7 cm wide, 12 cm long and 7 cm high you will need enough glass to fill a volume of 588 cubic centimetres.  As the specific gravity of glass is 2.5, you multiply the cubic centimetres to give the weight required in grams — in this case, 1470 gms.

If you wanted a 6 mm tile of 100 mm square you would need 150 grams of glass.

To make a 1 cm slab of the same size you need 250 grams of glass.

To make a billet of 5 cm by 10 cm square you need 1250 grams of glass (this is pretty close the the maximum that can be loaded in a 12 cm diameter Pot).

To make a small sample billet of 2 cm thick by 4 cm by 8 cm you need 160 grams of glass.

A billet or pattern bar of 5 cm by 10 cm by 5 cm needs 625 grams of glass.

Supporting Overhangs on Moulds

In general, the blank should be no larger than the thickness of the glass over the mould. So a 6mm blank would have no more than 6mm overhang.

In the case of steep sided moulds, the glass should be entirely within the mould to avoid any hangup on the edge, leading to uneven slumps and needling on the edges.

But, if you need the glass to be the size of the mould, you can make a collar to go around the mould, which will support the glass while it begins to slump into the mould.

Make a donut shape that will fit around the mould (whether round, oval or rectangular) and extend beyond. Support the collar on kiln furniture to be as high or slightly higher than the top of the rim of the mould. This makes a kind of drop out ring, allowing the glass to drop into the mould.

Donut ring suitable for placing around a circular mould

This arrangement is suitable for placing around a mould of the same diameter as the interior of the ring

Make sure that the collar is well covered with kiln wash to ensure the glass can move along the fibre board. This includes both the surface and vertical edges of the collar.

As the glass softens and begins to fall into the mould, the glass at the edge does not have the weight to bend down and so raises off the collar and begins to slip into the mould.

And finally, you need to ensure that the mould is not so steep as to trap the glass inside. This is more of a concern on steel with its greater expansion and contraction than ceramic.

A steel mould likely to trap the glass inside with its vertical sides

Super Glue

Super Glue

by Morganica

There are multiple cyanoacrylates (superglues) on the market, and they will give very different results. Gel superglue formulations usually have some type of rubber or fumed silica additive to make them thicker, and the additive usually doesn't burn out. That's probably where the "superglue leaves a mark" originates. Usually the cheapest possible superglue is best for temporary glass holds because it'll mostly be additive-free.

The glue will burn out around 700F or so, so it shouldn't be used to position the glass against gravity. I disagree, however, that it should never be used. I buy cheap superglue by the carton and use it in everything from temporary casting assemblages to making glass boxes for frit panels to tack-fusing. It is the best way I know to hold wobbly pieces in place until you can assemble the rest of the glass around it.

Some tips for using superglue:

• You are more likely to get whitish residues if you let moisture get to the superglue while it's drying, so keep the glass surfaces as dry as possible and don't put a superglue-assembled piece on a wet kiln shelf.

• Always try to put the glue under opaque or dark glasses, just in case something goes wrong.

• Use the smallest amount possible. Don't flood an area with glue and lay the glass on top - that will almost always leave too much glue on the glass. Instead, I assemble the glass and put a drop of glue right where the two glasses join. Capillary action sucks just the right amount of glue into the joint.

• If you wipe excess glue away with acetone, be careful about which acetone you're using. Some types (such as nail polish remover) can have additives that leave residues on the glass and make the problem worse. If the glue is in a readily accessible area, it is usually better to wait for it to dry, then peel it off the glass with a razor blade. Only use acetone where there's texture or something else that makes the glue difficult to remove. And in any case, don't worry much about removing superglue right on the surface--it will burn off.

• Superglue joints will NOT support the weight of your glass, i.e., never, ever lift your assemblage by a superglued-on piece of glass. Common superglue is actually a lousy glue for glass--which is why it works as a temporary hold.

Disguising Joints in Fusing

One advantage of fusing over leading or copper foiling is that shapes impossible to cut as a single piece can be made from multiple pieces. However these joints often show up in the finished work.

You are always more likely to have the joints show when the cut coloured glass is on the bottom. The infra-red heat of the kiln elements goes through the clear glass to the coloured below, allowing it to soften first. As the glass underneath softens and pulls in, it allows the top glass to sink into the space. Upon cooling the seam is kept open even sometimes showing a clear line at the joints.

Putting the clear as the base and the jointed pieces on the top has a better chance of having the joints fully fuse together. There is no glass above to spread the pieces apart.

When you need the joints to be concealed, you can put a line of powder the same colour of glass over the joint. This line should be slightly rounded above the surface along the joint to account for the reduction in volume as it fuses. When it is two colours meeting, using powder of the same colour as the darker glass is most successful.

Fusing to a contour fuse for 10 minutes is normally hot enough, but taking the piece to a flat fuse – again for 10 mins - will certainly be enough to fully melt the powder into the joint.