Initial Heat Up Rates

Example of a graph of a heat up for industrial purposes

There is quite a lot of information on the annealing cool rates, but not so much on the initial heat up rates. This is probably because the cooling rates are more critical than the heat up. But everyone knows that you can heat the glass up too quickly for its thickness.

My experience leads me to suggest some heat up rates to 50C above the annealing temperature for circular and nearly square full fused pieces. These have worked for me, but of course, may not work in all kilns.

6mm heat up at 160ºC/hr

12mm heat up at 110ºC/hr

19mm heat up at 50ºC/hr

25mm heat up at 30ºC/hr

In general, these heat up rates are no more than twice the initial annealing rate for the relevant thickness.  That is, the initial anneal cool for 6mm is 80C/hr; and for 12mm is 55C/hr.  When you get to 25mm, my initial anneal cooling rate is only 15C/hr.  So you can see how the doubling of the initial anneal cool works.

Tack fused and pointed pieces require much more careful heating because of the differing thicknesses within the piece, or the relative narrowness of one end or area in comparison to other areas. The suggestion is that the heat up for these should be at the rates suitable for items at least twice as thick as the thickest part of the tack fused or tapered piece.

Grinder Bit Height

If your grinder bit is too low or too high the diamond surface will not grind the whole of the glass edge. This can lead to chipping of the surface of the glass at the edges.

Example of top of bit almost too low for the glass

A good practice is to start with the bit as high as possible to allow for differing thicknesses of glass. As high as possible is with the bottom of the diamonds just below the platform of the grinder. This will ensure that you can deal with varying thicknesses of glass without immediate adjustment. You can then lower the bit as it wears.

Example of nicely adjusted bit

Of course, you need to ensure there is adequate water reaching the grinding bit to avoid overheating the glass, and to keep the dust from grinding from getting into the air.

Mobile Glass Storage

Sometimes people consider placing their glass storage onto wheels.  It is better to avoid wheels on glass storage for several reasons.

You would need heavy duty wheels to cope with the weight. Glass is 2.5 times as heavy as water, so it does not take much volume to make a really heavy glass case.

Example of mobile glass storage

You do not need to move all your glass at once - a piece at at time is all that is needed. It will be safer, and in the end easier, to build your work bench nearer the glass.

There are risks breakage while moving. The heavy glass store will vibrate the glass within the storage, and any snags or obstructions while moving the glass will increase the risk of breakage.

The structure can become too heavy to move and so defeat the original intention.

Antiquing Sandblasted Glass

Sometimes a sandblasted area appears too white when finished. One method that can be used to tone down the whiteness is to use low temperature glass stainers' enamel.

The low temperature enamels cure at temperatures between 530C and 580C depending on the type and manufacturer. At this temperature the glass is unlikely to change its shape. The jewellers and ceramics enamels fire at higher temperatures and are not suitable.

Rub the dry powder into the sandblasted area with a cloth or your fingers. The advantage of using the powder dry is that it will not stick to the smooth areas, although you may need to brush it out of any depressions in fused glass.

Fire the glass to the minimum temperature for the enamel, but for S96 or Bullseye try to stay below 540C. This temperature will fix the paint to the glass, but not change the shape of the sandblasting. Float glass will not change if you go to 580C. If you go to higher temperatures, you will go toward a satin effect and finally a smooth surface.

This technique has the advantage of being able to introduce a subtle colour tone to the sandblasted area. This enables you to match older glass that may have a slight colour cast from the glass or materials it has become encrusted with, such as nicotine.

This method requires testing to get the right levels of colour, and the temperature to balance the fixing of the enamel without changing the sandblasted surface beyond your choice. So you need to prepare several samples noting the amounts of enamel and temperatures used.

This has been successful for me when replacing broken sandblasted door panels that need to match the side lights. It removes the excessive whiteness of the new panel and can blend to match the colour of the originals.

Assembly of Circles and Irregular Shapes.

Leading

Circles and ovals as well as irregular shapes need significant perimeter support as leading often involves sideways pressures to fit the lead to the pieces of glass. Thus there are two main methods of support .

You can cut supporting pieces of glass to place around the perimeter. These need to be cut to the outside of the perimeter cut line. These supports must be in at least two or more pieces to enable the came to be put in place progressively. You then assemble the perimeter lead into it and continue to lead up as normal. The perimeter support can also be made from thin plywood or similar materials. This can be useful if the shape is to be repeated.

A simple means of supporting irregular shapes while leading is to place a number of nails around the inside edge of the cut line. There need to be enough to support each piece of glass with at least two nails. So you may need to add more nails to the initial set up. Build the panel without the perimeter leads. When the interior is assembled, put the perimeter came around the panel. Ensure the fully leaded panel fits within the dimensions of the opening. Then solder as normal.

Copper foil

The above methods can be used, but are often a bit heavy duty for copper foil processes. Instead of glass, timber or nails you can use more easily cut materials. Such things as stiff double walled cardboard, foam board etc., are suitable for light duty. You can cut the complete shape from these materials, but only good if no pressure is used in fitting the pieces.

If you are likely to repeat the shape and size, you can use plywood or similar materials. Build inside the shape and remove it when the whole is soldered on the first side.

Disguising Joints in Fusing

You can use powder or fine frit to conceal the joints in fusing. This is most easily done before the first firing.

Fine frit can be made from the off cuts from your prepared glass, or you can buy powder of the correct colour. Where two colours meet use the darker or denser colour of frit or powder over the joint. Push the frit into place with a brush or stick to form a regular edge. You should heap the frit over the joint to allow for the reduction in volume when fused. This will not work as well on tack fusing as it does on full fused projects.

Example of frit "painting", the principal of which can be applied to glass pieces

An example of how frit can be brushed around glass pieces

Placement of Pieces for Firing

Placing pieces in the kiln, especially in oval and side fired kilns, is not about filling the kiln completely. Kilns have hot and cold spots, and the arrangement of the elements can have an effect too.

The first thing to determine with a new kiln - and immediately after any alterations to the kiln - is where the hot and cool areas of the kiln are. There is an extensive guide to this on the Bullseye site. In short, the method is to place strips of glass on short kiln furniture all around the kiln at the level(s) you will be firing. These strips should be of equal size and the kiln furniture the same distance apart. Take the temperature slowly up to slumping temperature. Observe when the visible glass pieces begin to slump. Let that continue until they are about half way down. Then proceed to the anneal. When cool you can open the kiln and see the areas where the glass has slumped most – the hotter areas – and where it has slumped least – the cooler areas. This will give you information on areas to avoid if you want an even finish all around the edges.

If your kiln is side fired, you need to consider the shelf placement in relation to the elements. The best arrangement is to have one element below the shelf and the shelf between elements so the radiant heat is not directly onto the edge of the shelf as that may lead to breaks.

Put glass on the shelf as centrally as possible. If the glass must be near the elements, baffle the glass from the direct radiant heat from the side elements.

Glues in Kiln Forming

Glues have two major uses in fusing. One is to stick things together after being fused (cold fusing). The other is to hold things together before fusing.

Holding things together while preparing the piece to be transferred to the kiln is a major use of low tack adhesives and glues. All of these burn off a lot lower than the temperature at which the glass begins to stick together. So, if you are gluing overhanging pieces, for example, they can move after the glue has burned off.  If you are assembling pieces that will not stay in place while you are putting it together, glue will not help get the final result you want.  If you are gluing to keep things stable while you move it to the kiln, you may find everything is ok.

However, glue tends to boil off if the temperature is raised too fast. During this process, the effect of the boiling will move the glass pieces that are most unstable. This also occurs if you use too much glue. You should only use as much as will stick the pieces together. Also too much glue leads to black spots and sometimes bubbles between the layers of glass.

The adhesives commonly used are the Bullseye product “Glastac”, Elmer’s glue, diluted PVA - or school - glue. All of these take varying times to dry and hold the glass pieces in place. So, a popular alternative is hair spray. This is a lacquer which dries almost instantly. It provides a thin film of adhesive and burns off in the kiln with no residue. You should use the varieties with no additives.

Glue most often leads to problems or unexpected results, so several ways have been used to achieve the desired results.

One way to deal with unstable components on small pieces is to make a large piece with a repetition of the design and cut it up after fusing in to the sizes you want.  Clean the pieces very well, and then fire them again to at least fire polish to remove any cutting or grinding marks.

An alternative to using glue, especially at the edges where the pieces are likely to move, is to use dams. My practice is to make the dams slightly taller than the unfired piece and line with fibre paper. I put 3 mm fibre paper against the dam, and thinfire against the glass. Both of these should be 3 mm narrower than the final height of the fused piece will be. This is to allow the glass to make a rounded edge as it will not be able to stick to the fibre as it sinks down to its final height.

Bullseye hot dams as an example of damming

Another alternative to using glue is to fire the piece upside down, so that the pieces do not have to be supported. This does require some planning and forethought. You can draw the design in reverse on thinfire, using different coloured pencils for the various layers to help in building the piece up in reverse. You then cap the assembled pieces with the piece that will become the bottom. Take the whole to a tack fuse. Then clean very well to remove any residues from the shelf. It is possible to sandblast and then clean to make sure there are no residues left. Of course this is not possible if you are using dichroic or iridised glass. Also note that iridised surfaces and thinfire do not get on well – there is extreme pitting in the iridised surface. 

Example of pieces glued and ready for the flip

 Once the piece is cleaned, fire again to get the desired surface texture.

Cleaning a piece after first firing

Bones as Inclusions in Glass

The major components of bones are calcium and organic materials making up the marrow. If the bones are not old and weathered a very bad smell will be produced. The organic material will cause bubbles. Finally, it takes a long time to burn out the marrow, so it is best to use bones that have weathered for a number of years.

Calcium “erodes” during firing, so fine and thin bones will leave a shadow of ash (or a big bubble if there is not a long bubble squeeze. The bone has to be encased or trapped by the glass as it will not stick permanently to the glass on its own.

It can make dramatic shapes if the bones are arranged in novel ways to represent other things. The whole of the bone does not need to be encased, as the thicker parts will be strong enough to support themselves.

Hanging Sun Catchers

Unless you are using some manufactured system or a frame, the most frequent way to provide hanging points is to create a loop from copper wire.

Hangers should originate in a solder bead that goes some way into the piece. The loop's tail should lie a significant distance into the solder line to ensure it does not pull the piece apart. If this is to remain invisible, some planning will be required to allow the small extra space between the foiled glass.

The loops for hanging a piece of any size should not be soldered to the perimeter foil without reference to the solder bead lines, as the adhesive and foil are insufficient to hold the weight of the piece without tearing.

Here the hanging loops could have been moved just a little to engage with the solder joints at the left ear and at the tail to make stronger hanging points

Here the hanging points are at the solder joints giving strong hanging points

Reinforcement of free hanging or projecting elements can be done by placing wire around the piece with a significant excess going along the perimeter in both directions. The supporting wire can go into the solder line, if it is a continuation of an edge of the free hanging piece.

In this case a twisted copper wire around the perimeter gives strong hanging points

The strongest method is to wrap the wire around the whole perimeter of the piece. Choose easily bent copper wire. This will be pretty fine, but when soldered, will be strong enough support the whole piece.

The hanger can be made by leaving a loop of wire free. This way you can hang from any convenient place on the perimeter. This loop can be made by a single 180 degree twist in the wire, or by bending a loop into the perimeter wire. In all cases you will need to tin the wire to blend it with the rest of the piece.

This perimeter wire can be simply butted at the start/finish of the wire. It could be overlapped, but this is unnecessary on any piece where this method is adequate for support. The start can be at the top or bottom, although I prefer the top, so the wire is continuous from loop to loop. The reason for continuing beyond the loops is to provide support to all the edges of the sun catcher.

This single point hanger is at the strong point of the piece

The left hanger is strong, but the right is weaker than if it had been attached to the right of the body

This piece needs wire around the piece, especially to stabilise the tail and ears