Bubbles Between Layers – “Flip and Fire”

Another approach to avoiding bubbles is to plan on two firings. This works well for pieces that have multiple layers, with glass or other inclusion in the middle.

For the first firing, put the middle pieces flat on the kiln shelf with one layer of glass on top. Take this to at least a tack fuse, although full fuse temperature is better as there should be no remaining gaps for air to be trapped within. Now turn this over and clean it well. Place this part in the kiln with the middle layer up. Place the top layer over this piece – now right side up – and take to the full fuse. Remember that now you are firing a thicker piece than in the first firing so take the temperature up more slowly.

This is most often applied to three layer pieces, but in principle can be applied to any number of layers.

Using baffles
Supporting the edges
Design elements
Arrangement of glass sheets

Bubbles Between Layers – Baffles

Bubbles are often caused by the edges of the layers sealing before the air can escape from between. This frequently happens in side fired kilns, and top and side fired kilns.

Set up heat baffles around the edges of the sheets being fused to decrease the chance of the edges getting more heat than the centre and trapping air between layers. The baffles can be made from kiln furniture, strips of fibre board, cut pieces of old kiln shelves, etc. - anything that will witstand the top temperature.

Arrangement of glass sheets
Designing for fewer bubbles
Edge supports to reduce bubbles

Bubbles Between Layers - Supports

A common way to reduce bubbles that appear between layers of glass is to support the edges of the glass allowing the middle of the top sheet to sag before the edges so pushing the air in front of the collapsing glass.

You can do this with small beads - especially useful for large glass sheets. These beads are prepared in advance by firing small pieces of glass during a previous fuse firing. The glass draws up into a bead-like structure. You place these beads around the edge of the glass sheets. Use glass that is the same colour as the base glass to avoid strong colour spots in the finished work.

Make sure you advance the temperature slowly enough to allow the glass to slump from the middle outwards, allowing the air to escape. Note that even clear beads will leave a trace, so design your work to take advantage of these faint marks.


Another method is to put small pieces of frit every few centimetres around the edge of the bottom piece of glass. Place the top piece of glass on top of these spacers. When fired, the middle of the top sheet will sag first and the area of contact between the two sheets will spread from the middle pushing the air out as it goes, just as with the beads. But the evidence is not so marked as with the use of beads. However the frit is not so useful on large pieces.

Design factors
Arrangement of layers
Using baffles

Fibre Papers

As there always is concern about the health effects of ceramic fibre paper, the report I prepared for a supplier may be of interest.  It can be found here.

Bubbles Between Layers - Design

Design your work to minimise the possibilities of trapped air.

One way to do this is to use strips. Lay thin strips of glass on edge and fuse these together, instead of layers stacked on each other.

Another is to design work with many smaller pieces, rather than large ones. These create more pathways for air to escape.

Some advocate cutting the bottom layer in several strips to allow the air exit spaces from between the glass layers.

Note that all these methods leave marks of where the edges of the cut glass was, so they need to be planned to fit with the design.

In general terms, you need to think about how the air will move out of the piece. Are there places where there is no escape for the air? Allow a channel for the air to move from the centre to the outside.

Glass arrangement considerations
Supports
Using baffles

Bubbles Between Layers - Arrangement

There are a variety of ways to minimise bubbles between layers.

You will have noted that there are smoother and rougher sides to the glass. Putting the two rough sides together will increase the number of small bubbles in the finished piece. But the opposite –smooth to smooth – does not produce the smallest bubbles. The fewest small bubbles are produced when one smooth side is touching the rough side of the other layer of glass. This “roughness” allows any air to find a way out. Smooth to smooth tends to produce fewer but much larger bubbles.

Design factors
Supports during the forming stage
Using baffles

Attaching a Spider to a Lamp

To get the maximum support from the spider, bend the legs to fit the shape of the lamp. Then clean the spider assembly well with steel wool, or if particularly dirty with fine sandpaper, until it is bright again.

Apply flux and run a film of solder onto the legs where they will be in contact with the lamp. This will give you an indication of the amount of heat required to solder the legs to the lamp. It will also make it easier to fix the legs to the lamp once the appropriate temperature has been achieved in the legs.

You need to use the full power of your soldering iron, as the brass soaks up a lot of heat. If your iron is small you may need to change to one with higher wattage for this work.

As the heat is transferred quickly to all parts of the spider, wear gloves or hold with pliers while soldering.

Hanging Panels from the Borders

The hanging method for copper foiled and leaded panels depends to a large extent on size and weight. The larger and heavier the panel is, the stronger support that is required. Smaller light weight panels are not usually a problem as the solder lines and joints can take the weight. Leaded panels require more support than copper foiled panels once they are over a few kilos, or are anything but rectangular.

For heavier panels you need to have stronger supports than just the perimeter calmes or solder beading.

For rectangular panels you can use a stronger edge came such as zinc or brass. Solder this to each meeting joint throughout the length of the panel. This distributes the hanging stress across the panel more effectively. Simple soldering at the four corners of the calme will rely on just those solder joints to carry the whole weight of the panel.

You need to make the vertical borders of single pieces so that you are not relying on the strength of a single solder joint at some point along the side. Although the joint may be strong enough at present, it must stand up to the weight of the panel over a long period of time.

All this relies on secure attachment of the hanging hooks or wire

Glass Weaving

In its essence, weaving is creating a series of strips with waves, moving alternate ones a half step along and inserting straight strips into the channels provided.

It can be as sophisticated as you can devise, but remains the shaping of a series of strips through which other, straight strips are threaded at right angles to the shaped ones. As in cloth weaving, there are a great many variations that can be devised.

You can use a variety of material to slump over – covered steel pipe, cut up kiln shelves, brick, fibre board, etc. - but you must remember that you will need at least 10mm height, as the upper and lower pieces of glass are 3mm each and the one inserted will also be 3mm, leaving only 1mm tolerance. You also need to ensure the material slumped over is far enough apart to accept the width of glass you will be threading through. If you are using 20mm strips, you probably will need at least 25mm intervals between the slumping strips. Make sure they are parallel as well as evenly spaced. You will need to soak at slumping temperature longer than for a simple shape, as you want the slump to be close to vertical.

When threaded, you can tack or full fuse the piece and subsequently slump it if desired. The amount of space between the “threads” will depend on the steepness of the slump. If the slump is too deep you will find the lines of the “threads” will be uneven and may even fold over one another.

So this is yet another area of kiln forming that is simple in principle, but requires a lot of experience to get a really good looking piece at the end.

Drilling Glass, 2

Using a Drill Press

It is best to have a drill press if you are doing a lot of drilling. It provides a stable drilling action and the pressure on the bit can be controlled.  It is important to ensure the bit is running true without wobble. The drill press should have instructions to help correct any untrue running of the chuck.  Make sure the drill bit is secured firmly.  Core drill bits are easier to keep true, as they normally have a threaded fixing.

With a drill press, you can drill continually until the hole is completed, or until a white paste or dust begins to appear. This indicates the drilling is being done dry and will in a few moments heat up the glass too much. When the white paste appears, back out of the bottom of the hole a little to allow water to flush the glass out. Then continue.

Keep a firm grip on the glass being drilled. Maintain its position, especially if you are intending to back off intermittently to allow water to the bottom of the hole. This enables you to get back into the hole without scratches.

If possible, submerge the piece. But if that isn’t possible, just squeeze a little puddle of water on the surface and watch it swirl around. You can see if it is pulling ground glass out of the hole by watching the circulation. Placing a plasticine or clay dam around the drill area will keep the water confined.

Don't push down any harder than you comfortably can with the tips of your fingers. Keep it steady. Listen for the sound of diamond grinding glass

If the core gets stuck in the bit, knock it out with some stiff wire or a nail. Always remove each core right after drilling. They are very difficult to remove if there is more than one in there.

Every diameter drill bit has an optimum drill speed. The smaller they are the faster the speed required. Based on what the manufacturers recommend, a Dremel running at top speed is way too fast. When using a Dremel for drilling glass, slow it down with the speed control. Drill presses do tend to be on the slow side for glass drilling so it takes a bit longer, but there are big advantages in other respects.

For other tips on glass drilling see:

Keeping things wet
Using a drill press
Drilling with a Flushing Head
Avoiding chipping
Drilling holes with copper tube and grit
Drilling tools
Drilling glass without a drill press
Hole Placement
Drilling speeds for diamond bits in glass