Rods in Glass

Inserting large rods into glass

Normally large diameter rods are incorporated into a glass piece for display supports. However, rods over about 2mm will break the glass when incorporated in the fusing. This means that rods to support the glass that will be of sufficient diameter for the strength required cannot be fused into the glass.

You can use a rod of the same - or slightly smaller - size if you wrap it with thin fibre paper to cushion the differentials in expansion and contraction. When the glass has cooled, pull the rod out and clean out remaining fibre paper with water and a pick. 

Alternatively, you can use fibre paper to create a void of similar size to the wire or other metal support that will be inserted.

If you find that you do not like the hump that the rod or fibre paper create, you can use a third layer between the top and bottom.  Cut the glass to leave a channel of the length you want.  This means that the intermediate glass will need to be in at least two vertical parts, with half the channel width taken out of each side.  You can cut the glass in three parts horizontally.  The main piece is cut to a length minus the depth of the channel.  Then two strips are added to the bottom which leave the desired width. Finally, put the top layer on.  

Alternatively, the middle layer can be the design, with the bottom and top being clear or any other colour desired.

Once fired, clean out the channel and glue the rods into the glass. Silicone is a good all purpose adhesive that will allow a bit of movement.

Slumping Single Layers

Slumping small single-layer pieces

A different set of conditions are formed when slumping small pieces. It takes more time or heat to slump small pieces, because there is less weight to pull the glass down into the mould. Because there is less weight in a single layer than in a two layer fused piece, more time is required for the glass to conform to the mould. The exact amount of time and heat will have to be observed for each kiln and kind of layup.

The alternative strategy is to increase the heat. This can work, but leaves more mould marks on the finished piece. The increased temperature causes the forming to occur much more rapidly and may lead – if not closely observed – to over stretching or “bubbles” at the bottom of the mould.

If you are not concerned about the exact nature of the shape of the piece you can fire pretty fast and high. This will produce a curve, but probably not good conformation to the mould. Even if there is good conformation, there will be significant marking of the back of the glass.

If you want a relatively unmarked and slumped piece, you need to change strategy. You can take the glass up in temperature rapidly to ca. 600C and then use about 40C/hour to 677C. Begin watching from about 635C. This slower advance in the slumping range gives better control of the slump. Slower rates of advance give less distortion and less movement within the mould. It also provides a less marked piece, because the slumping is done at a relatively low temperature.

When the piece achieves the shape you want, record the temperature and advance to the next – cooling - segment. The recording of the temperature will give you information for future slumps of this nature.

On any slump that is new to you, either in terms of the mould or the thickness and size of the glass, you need to make records to help determine the optimum combination of conditions. You should of course, record each firing anyway.

You will find more information on factors affecting slumping at these locations:

Weight/thickness

Mould sizes

Mould shape and detail

Uneven slumps

Firing speeds

Rounded edges on slumped pieces

Often people want to know how to get rounded edges during slumping of a single layer piece, especially when trying out techniques with their spare art glass.

Achieving a rounded edge on a slumped piece is a combination of temperature, thickness, larger top layer and cold working.
Rounding of the edges of a piece of glass occurs at tack fusing temperatures, which are beyond the slumping temperature. It is possible to take the glass to a tack fuse within the mould as long as you are prepared for some consequences.

More mould marks are evident on the bottom of the vessel.
Mould life is reduced.  You get a lot more mould marks on the bottom of the glass because the bottom of the glass is softer than in a standard slump. These marks will be directly related to the surface texture of your mould.

You need to re-coat the mould before the next slumping to avoid the kiln wash sticking to the glass.  Ceramic based moulds last a long time if fired below 680C. But numerous firings at tack fusing and higher temperatures increase the possibilities of glass sticking to the mould and occasionally, thermal shock. If you insist on tack fusing in your mould, you need to renew the separator each time, as the kiln wash breaks down at tack fusing temperatures leading to it sticking to the bottom of the next piece you fire in that mould.

You may get an uprising at the bottom, as the glass slowly sinks down the mould and pushes the glass up at the bottom in any mould other than a simple, shallow shape.  

These things indicate that it is best to tack fuse first and then slump at the lower temperature.

Of course the best result can come from using 6 mm of glass, with the top layer 6 mm larger than the bottom layer. This allows the upper layer to sink over the outer edge of the lower one, giving a rounded edge with no sign of any differences between the two layers.

Cold working solution 
If however, you want to work with a single layer, you need to realise that the edge will be the same when it comes out as when it went in. So you need to cold work the edge before slumping. You do not need sophisticated machinery to do this. A few diamond grit hand pads will do the job. Start with one at about 100 grit to shape the edge. Make sure you keep the pad and working surface damp. If you begin to get a white paste appearing, you need more water.

After shaping the edge satisfactorily, take a pad of about half the grit size (twice the number) and begin the smoothing of the scratches created by the shaping. When finished with one grit move on to the next. You can use a paint marker to help tell when one grit is finished. 

This process will give a sheen that will change to shiny during the slumping.
  

  

Mould preparation

The preparation of a mould is more than adding a separator to it, although that is essential.

The glass will always take up the mould texture if it touches. So start with smoothing the mould itself. You can do this by wrapping very fine wet and dry sandpaper round a flexible sanding block. You can use this wet or dry, although wet is more healthy. When you get the desired surface texture you can let the mould dry or immeidately apply kiln wash. When this is dry you can again sand the kiln wash but with dry paper only this time and very lightly. Alternatively you can ball up nylon stockings to provide a soft abrasive action. If you sand through to the mould surface, simply apply the kiln wash again.

You can minimise mould marks by slumping at the lowest temperature which can achieve the slump you want.

If you want to avoid mould marks altogether, do an aperture drop and cut off the rim. There is no mould contact with this technique, so the glass retains the surface it had in fusing.

Moretti Rods

Information on Moretti/Lauscha specifications:

Coefficient of Expansion (C.O.E): 104

Strain Point: 448C

Working Temperature: 926C

Annealing Range: 493 – 498C


Softening Point: 565C

Rods per pound: 15-17

Rod 
Length: Approx 1metre

Rod Diameter:5-6mm

from yglass.com

Mesh Melt Grids

You can construct your own grid rather than relying on the barbecue manufacturers to use high grade stainless steel.

You can buy stainless steel rod and cut it to the lengths you need for your kiln or project. Push them into soft fire brick at each end. Do the same with another pair of fire bricks and put one pair at right angles to the other.

In this way you can vary the intervals to suit yourself for different projects.

You could also put the rods on strips of mullite and weight the ends so the rod does not roll about while assembling them or when firing.

Alternatively, you can buy the grids available for various purposes. Not all are high grade stainless steel, but normally the spalling happens at a temperature that is low enough to avoid incorporation with the glass.

Glass bending

The purpose of glass bending is to achieve a mark-free curved surface. This is usually done with glass that has smooth surfaces - normally clear but it can be applied to any smooth coloured glass too.

There are at least two methods – moulds and free bends.

Mould

Using a mould enables you to achieve the shape you want with the least observation. However, you need to be careful to use the lowest temperature to achieve the shape to avoid marking the glass. You also need to measure the outer circumference of the original shape, as you are bending, not stretching the glass as in most other slumping operations. A mould is most useful when the shape is not a radius curve. Metal can work very well and because you are using low temperatures, a dusting of alumina hydrate or talc will act as an effective separator. Of course you can use ceramic or fibre as a mould too.

To be able to use the low temperatures required, you need to take advantage of the weight of the glass.  This means the glass needs to fall into the mould, not drape over it.  Relatively fast rates of advance can be used, as you are normally bending one layer of glass.  However from the annealing point upwards the rate should be slowed to allow all the glass to heat throughout and enable the bend to occur at low temperatures.  Observation will be required to determine when the bend fully conforms to the mould.

Free drop

A free drop is similar to an aperture drop, but using a channel rather than a bounded opening. To do this arrange a channel of the appropriate width plus 10mm to allow the full curve to form at the edge. Kiln shelves that are cut into strips, or lengths of fibre board - both supported on kiln furniture - will provide a good channel. Apply kiln wash to horizontal pieces forming the channel or cover in fibre paper. Place a witness at the appropriate height to ensure you can see when the glass has reached the depth/curve required. Initially, this seems to be extra work, but the expense of making a metal mould far outweighs the time taken to set up a free bend for simple radius curves.

Temperature

The temperatures required for glass bending are 40C-80C above the annealing point of the glass. For simple curves start with the 40C above annealing whether using a mould or channel . If after two hours, nothing has been achieved, advance 10C and soak for another two hours. Repeat as necessary. Lots of observation is required. Remember that the wider the aperture and the thicker the glass the less time and temperature is required to achieve the result. After the first bend you will know the combination of temperature and time required for the depth and width of any other piece like this.

Stopping the bend

If you are using a mould, you simply advance to next segment. With a free drop you can also simply advance to next segment as the movement is so slow. But if it is a deep bend - more than a simple radius curve - advance to next segment and open kiln until annealing temperature is reached. Then close the kiln and anneal.

Anneal for the thickness of the glass. No special annealing is required.

For the free drop, when cold, cut off the excess to the size required for the opening.

Freeze and Fuse

The object of this technique is to make a shaped piece without use of a refractory mould. It is applicable to small items.

You can use jelly, soap, candle, etc. moulds. They can be rigid or flexible. They should be without undercuts and have a draft, which is why jelly, soap, and candle moulds are so suitable. Be careful of the size, as a large amount of frit can be required even for a small mould.

Some people use only powder for this process. I use a 50/50 combination of powder and fine frit. You can use clear frit with powder. If you do so, you need to measure out the appropriate amounts.  Then put the frit and some water into a container with a closure. Close and shake to wet the frit. Then add the powder and shake again to ensure the powder adheres to the frit. Once thoroughly mixed, add more water to make a thick slurry.

Pack the mould with the mixture. Then using absorbent paper towels firmly pat the contents of the mould as dry as you can. Place the mould in the freezer for at least a couple of hours, or for large ones overnight.

When frozen, remove from the mould and place on the kiln shelf. The shelf needs a separator which can be kiln wash or fibre paper. Some leave the piece to thaw out and some more of the water to evaporate.

Whether you fire immediately or let the piece thaw and evaporate, you need to fire slowly to 100C and soak there until no more moisture is evident to avoid creating pockets of steam that will blow the piece apart. After that you can fire as normal for an initial firing of a two layer piece.

The piece will shrink a bit during the firing, but it is safe to anneal for the original thickness of the frozen piece. If you have a large piece or one with lots of variation in thickness, you should use at least the next thickness up from the Bullseye tables for annealing thick pieces. Sometimes you should use two steps up.

Further information is available in the ebook Low Temperature Kiln Forming.

Fibre Board for Kiln Shelves

Some advantages of fibre board:

• It is light weight making it easy to move.
• Fibre board has very good cooling characteristics as it doesn't hold the heat the same way a mullite shelf does.
• The board is fragile, but with care can last years.
• Fibre board shelves do not thermal shock as ceramic based shelves can.

Considerations for use:

• If it will be moved it needs hardening.
• It needs repeated sanding and hardening for a really smooth surface. Alternatively you can smooth on something like batt/kiln wash or alumina hydrate for each firing.
• It needs to be supported on kiln posts at 100 mm intervals.

You need to use dust masks when ever working with fibre board.

Some disadvantages of fibre board:

• The board can warp over time even when supported every 100 mm.
• The board will warp over time if placed on the kiln floor.
• The board needs to be thick - at least 25mm, thicker for larger kilns.
• It can't be scraped clean of batt/kiln wash.
• Ceramic fibre board is possibly not much cheaper than mullite shelves.

COE

CoE

CoE is an abbreviation for coefficient of linear expansion. This has a very specific definition which is not widely understood.

The "linear expansion coefficient" is determined by a laboratory test which expresses the average expansion rate from room temperature to 300ºC. It ignores the more important range of expansion for determining compatibility for fusing and the expansion through the annealing and softening ranges as this graph shows.

It also ignores viscosity, an important element in determining whether glasses will "fit" each other on fusing. Not all "90" expansion glasses are compatible.

The two major factors in a fusing compatible glass are expansion characteristics and viscosity. These need to be in balance with each other within one piece of glass and with other pieces of glass with which they are combined. There are other factors of course, because glass chemistry is very complicated.