Gas Fired Kilns

Installing a Gas Fired Kiln

There are a number of considerations about the location of a gas kiln.

Air exchange

The kiln should be placed in an area where there is good air exchange. For a number of reasons, it is usually convenient to place the kiln near an outside wall – ventilation and canister storage are the two most important.

Ventilation

There should be a low level vent to the outside to allow air to rise for the use of the combusting gas. There needs to be high level vent for the gas to escape. Ideally a hood with powered extraction would be installed.

Space

As with any kiln, you need to have sufficient space around the kiln to avoid heating any flammable materials. In the case of a gas kiln you need to be careful to avoid storing any thing that might ignite above the kiln. You can put heat resistant materials around the kiln if desired.

Location and storage of gas canisters

The safest place to put the gas canisters is outside the building. This does mean going outside to turn the valve on the gas canister on and off. However this is the safest place, should any fire start in the building, because the fire brigade can find and remove it from the fire. This placing applies to both the in use canister and any other full or empty canisters.

Detectors

Finally, even with these precautions, you should install a carbon monoxide detector. Carbon monoxide has no smell and can overcome you very quickly. Even a few minutes of exposure can leave you feeling ill for days.  

Bottle Slumping

Placing

Production made bottles are not evenly thick all the way around. So they roll until the heaviest part is at the bottom.

One way to avoid any rolling in the kiln is to put the bottle on a level surface and let it roll to the heavy side. Mark the up side so that you can place it in the kiln with the heavy side down.

Some times though the seam lines – which will show in the final piece – are in the wrong place. To keep the seam lines to the side, you can place a small piece of 0.5 mm fibre paper against each side of the bottle at the base if you are working on a hard surface.  On a softer surface, the weight of the bottle or a gentle push will sink it into the surface enough to avoid rolling.

The distance to place bottles apart is important to know so they do not become attached when several are slumped together.

You can slump test bottles of various diameters to determine their final width.

However, if you wish to calculate the distances, The width of slumped bottles is approximately 1.6 times the diameter of the original bottle. If you want the base of the bottle to be flat too, the final size will be wider, dependent on the amount of glass in the base.

You also need to leave some additional space than this calculated distance between bottles, as the final size is narrower than the size when hot. You should leave at least an additional 10 mm to each side of the bottle.

So a 50 mm diameter bottle will become 80mm wide. You need an additional 10 mm each side, so the spacing from the centre of one bottle to the next should be at least 100 mm.

The surface that you are slumping onto is important too.

You can place the bottles on a sand base that has been dusted with kiln wash powder. This has the advantage of allowing a gentle push into the sand to prevent rolling. But it imparts a grainy texture to the back.

You can get a smoother surface by using whiting or sifted plaster that has been screeded smooth. This also allows the bottle to be gently pressed into the surface.

You can use fibre papers as separators from the shelf, but they are relatively expensive.

Kiln wash works very well. It can provide a very smooth surface, but if you want more texture, you can sprinkle some kiln wash through a sieve over the shelf.

Thick Uneven Pieces

Occasionally fused pieces come out of the kiln with one side thicker than the other. There are several things that need to be done for the present piece and for the future.


Level

First check how level your kiln is. The best for this is to begin with a check of the bed of the kiln. Check the level in four directions – left-right, front-back and the diagonals. If it is practical, wedge up the legs of the kiln to make the bed of the kiln as level as practical.

Then check how level your shelf is. Put in your shelf supports and then place the shelf on them. Again check with a spirit level the four directions. Place pieces of fibre paper under or on top of the supports to level the shelf. It is only after these checks have been made that you can consider firing your piece to help it return to an even thickness.  As part of your kiln maintenance you should check the level of your shelf at least monthly, if not every time you prepare to fire.

Variation in Thickness
Now that you know the shelf is level, you need to consider what the variation in thickness across the piece may be. The firing schedule needs to be more conservative than just for the thickest part. As the thinner parts will heat through more quickly than the thickest parts, you need to fire less quickly than you normally would for the thickest area. A rule of thumb – not always correct of course – is to add the difference of the thick and thin areas to the thicker and fire for that calculated thicknesses. This will make the firing schedule slower and so allow the thicker part to be the same temperature as the thinner. For example, a piece 6 mm at one side and 10 mm the other would have a difference of 4 mm. Add this 4 mm to the thicker 10 mm and then fire for 14 mm.

Temperature and Soak
You also need to consider the top temperature to use and the length of soak required. Glass flows relatively slowly at kiln forming temperatures. The conservative approach – one that allows further work if necessary – is to use the previous fusing temperature and extend the soak by at least twice the length of time on the previous firing, even perhaps to a couple of hours.

Bubbles
One thing that will happen is that the bubbles that previously were near the surface will rise and burst giving pin holes on this extended soak. So you should consider cleaning the bottom and putting the top face down on a separator between the shelf and the glass.   This will reverse the direction of flow for the bubbles. Few if any will break through the new top and there should be no pin holes when flipped.

Further Firings
When the piece is cool, check it for the even-ness of the piece all around. If it is not even enough, you will need to consider re-firing again. If you decide to do so, you should go no faster than the rate of advance as previously – probably even slower - but consider raising the temperature or extending the soak. Remember that achieving the heat work required at the lowest temperature is the guide line for kiln forming. So an extended soak should be preferred over a higher temperature, unless there are strong indications that a higher temperature is required.

Fire Polishing
Of course, you will now need to throughly clean the face down side and re-fire to fire polish the original top. The rate of advance should be the same or slower than the firing to even the thickness. Once you have achieved about 600C, a soak of about 30 minutes will ensure that the glass is thoroughly heated through. Then you can advance at a quick rate to the fire polish temperature with a soak of no more than a minute. This allows the surface to change without giving the rest of the glass time to begin to move.  Of course, a thorough annealing is required.

This procedure for re-firing  can be used when re-firing pieces for any reason. You only need eliminate the considerations on the uneven thicknesses.

Separators for Steel

Steel moulds, whether stainless or other steel, require separators as glass will stick to hot metal.

The preparation of steel is a little different from ceramic or fibre moulds. First the steel has to have the oil coating removed. This can be done by sanding, sandblasting or by heating to about 550C.

The separators can be just normal kiln wash, boron nitride, or fibre. When using kiln wash or boron nitride, the steel needs to be heated – about 200C should be enough. Too hot and the water will boil leaving gaps in the coating. If the metal is too cool, the kiln wash will drip. A little experimentation will be needed to find the right temperature for your purposes. Remove the mould from the kiln and brush or spray on the separator. Return to the kiln to heat up again and apply the separator again. Continue this until an even complete covering of the metal has been achieved. It does not have to be thick.

Fibre paper works best on cylinder or wave moulds with curves in one plane only.  Bowl shapes lead to wrinkling of the fibre papers and marking of the glass with wrinkles.  This applies to the Bullseye Thinfire too.

Just as on shelves and ceramic moulds and as long as you fire below 700C, you can use the coated mould over and over with no problems. Once you fire hotter than 700C, it is best to remove the old kiln wash and put a fresh coating on before firing again.

Sands for Texture

Firing on sand can give an even stippled texture. It can also provide immediate, free form textures and shallow shapes. You can use different sized grains for variations in texture size. So it can be a quick, responsive medium to give textures and shallow shape to the glass.

Although you can fire on sand, you must use a separator. When I fire directly onto the sand for texture, I dust alumina hydrate over the sand. People who watch me dust it through a sock have a laugh, but it does work to provide a fine layer of separator, so that any sand picked up by the finished glass can be removed as it is not fused to the glass. Sometimes it takes a bit of cleaning effort.

There are a number of sands that you can use, although all require a surface separator.

Silica sand is the kind of sand found on a beach and is the most commonly used sand. It is the most commonly used sand because of its great abundance and low cost. Its disadvantages are high thermal expansion and low thermal conductivity which requires caution in annealing the glass.

Olivine sand is a mixture of orthosilicates of iron and magnesium from the mineral dunite. Its main advantage is that it is free from silica, although a separator is still required. Other advantages include a low thermal expansion, high thermal conductivity, and high fusion point. Finally, it is safer to use than silica.

Chromite sand is a form of magnesium aluminium. Its advantages are a low percentage of silica, a very high fusion point, and a very high thermal conductivity. Its disadvantage is its cost.

Zircon sand is a compound of approximately two-thirds zircon oxide and one-third silica. It has the highest fusion point of all the refactory sands, a very low thermal expansion, and a high thermal conductivity. However, it is expensive and not easily available

Chamotte sand is made from previously fired clay. It has a relatively high fusion point and has low thermal expansion. It is the second cheapest sand, however it is still twice as expensive as silica sand. Its disadvantages are coarse grains.

Aperture Drop Placement

Aperture drops normally are placed much higher in the kiln than most work to get the greatest length of drop. This means that the glass is near the elements and so will be heated unevenly. It has been said that the heat evens out across the kiln approximately the distance below the elements that they are apart. So if the elements are 100mm apart, the heat will even 100mm below the elements. This constraint means that it is difficult to get the length of drop wanted and still have the glass heat evenly.

There are at least two things you can do to get more height. One is to take out the shelf and its supports so you can fire on the bottom of the kiln. This will give up to 50mm extra drop length.

The other is to go ahead and fire closer to the elements than is indicated for even heating. This will require radical modification of the heating schedules. [qv firing]

Aperture Drops Introduction

Aperture drops are apparently simple to do. But to have control of the process and to be able to get repeatable results is relatively complex. There are various elements that need to be considered when preparing to make one of these. The main technical considerations are:

The height of the drop from the shelf.

Material of the supporting ring or material.

Diameter of opening of the aperture.

Size of the blank in relation to the aperture

Initial firing speeds

Height in kiln and relation to the distance from the heating elements.

Observation of the progress of the drop.

Arresting the drop

Annealing and cooling.

Finishing the resulting drop.

The above instalments will discuss these in turn.

Removing Glass from Kiln Shelf

Care is needed when removing glass that is stuck to the shelf. You need to protect your hands with thick gloves, as any slip will cut your hands deeply.

For mullite and other ceramic shelves you can use a variety of tools:

If there is a small amount of glass in one or more spots, you can use a scraper or lead knife. The wider the blade is, the less chance there is of creating a big divot beside the stuck glass.

If the stuck glass is large or thick, you can use a hammer and chisel. Care is needed to avoid creating a bigger hole in the shelf. Use very shallow angle, almost parallel to the surface of the shelf to chip out the glass.

Diamond hand pads are useful to get the last bits smoothed out. You need to be careful of creating a low spot by working only in a concentrated area. One way of avoiding that is to use a slurry of grit and grind with large sheet of float glass. The area being covered is large and so reduces the danger of creating low spots. Remember you can get away with smoothing the shelf, not all the glass has to come out of the shelf. If the bits of glass are only small, it will not reduce the life of the shelf much, although glass tends to be corrosive to kiln brick and ceramic that it is in contact with.

If removing the glass has taken a significant amount of the shelf surface off, you can repair it. A temporary repair is to fill the divot with dry kiln wash and smooth it with a plasterer’s float or a piece of float glass. A more permanent repair is to mix a small amount of cement fondue with or without a little vermiculite. Smooth this level with the rest of the shelf while wet, as it is very hard after curing, which occurs at about 600C. If the mix is of cement fondue only, it will tend to reject the kiln wash, as it is more dense than the shelf.


Removing glass from fibre shelves in some ways is much easier, as the shelf material comes away with the glass. This does mean that repairs are always necessary. This can be done with a temporary fill of dry kiln wash or more permanently with a mix of 1 part cement fondue to about 6-7 parts vermiculite. This makes a less dense filler than cement fondue on its own, which would be too hard for fitting with the fibre shelf.

Disposal of Used Bullseye Thinfire

The main ingredients of Bullseye’s Thinfire are cellulose, aluminum hydroxide, fiber glass, and organic binders. It is predominately a nuisance dust and irritant.

Use a vacuum sweeper with a high efficiency filter and a bag rated for plaster dust. Also many vacuums with a HEPA filter system will be sufficient. Wrap the disposable bag in another -preferably paper - bag to avoid dispersing the dust when it goes into the rubbish.