Spider Web Cracks

 

Credit ASTM

 The nature of the cracks - and spider web describes it perfectly - shows an adhesion problem. It is not an annealing problem as that shows a single sinuous line with a hook at each end. It is not a compatibility problem, as that shows as cracks or breaks along the edges of the combined glasses. It is not a thermal break, as those show as breaks where the glass has separated to some amount.

 

Glaze crazed in a ceramic vessel

 

The cracks are exactly like crazed glazes on ceramic objects. And for the same reason. The glass is trying to contract more than the underlying ceramic. It is stuck to the pores of the ceramic and creates a crack where there is a slightly weaker part of the glass. These cracks in ceramic glaze propagate across the surface as it wears, or in the kilnforming case as it cools.

 

Glass puddled in ceramic

 Most usually it results from a lack of separator in that area of the shelf, or uncoated kiln furniture. It indicates either the glass has adhered to the shelf or mould, or (rarely with fusing glass) that the glass has suffered severe devitrification.

 

 

 Occasionally there will be the appearance of shards of glass. This will be where the glass has stuck to some particle on the shelf. Sometimes it can be a speck of something resistant to the temperatures we use in kilnforming that “grabs” the glass and breaks it into shards from that point as the glass cools.

 It is not the schedule that causes the breaks. It is in the shelf preparation.

 The shelf should be cleaned of all the kiln wash and lightly sanded down to smooth. It should then be coated with four thin layers of kiln wash painted in a different direction for each layer. No drying is necessary or even advisable. All kiln furniture must be completely coated with kiln wash.

 If you are re-using a shelf, it must be swept clean before any glass is laid on it.

 Crazing results from the glass sticking to the surface it is resting on.

 

Some additional information:

https://glasstips.blogspot.com/2019/05/kiln-cleanliness.html

https://glasstips.blogspot.com/2020/07/crazing.html

 

Large Tiles for Kiln Shelves

Pizza stone in use

People frequently wonder if other materials than mullite can be used for kiln shelves.  Mullite is used for its strength and very small expansion, even at high temperatures, as used in ceramics firing.  There are other materials that can be used in kilnforming of glass such as refractory fibre board, and ceramic pizza stones,  the best of which are made from mullite.  This post is about using ceramic floor tiles.

An unglazed floor tile, 11 x 11 inches

Structural Soundness

A major element in obtaining and using a floor tile is how sound it is.  Tapping the tile to determine whether the sound is a low toned ring or a dull thud is important.  There may be invisible cracks within the tile.  A dull thud is an indication that the whole tile has one or more cracks in it, or that it has not been fired high enough to completely vitrify the clay. A low frequency tone indicates there are no cracks and that it has been fired sufficiently high.

Flatness

The first thing you need to do is make sure the ceramic tile is flat and without undulations before using it. To test this, get a straight edge and move it along the tile to look for any slivers of light coming through underneath the straight edge. Any light or variation in the amount indicates depressions that can produce bubbles during the firings. Do this test at least twice at right angles to each other.  Take note of the depressed areas (or even possibly high areas) to know where these uneven areas are to work them out of the tile. 

You can do the above test in the showroom.  Another more accurate means of checking is more difficult to do in a sales area.  Place a line of dark powder, say black glass powder, and with a straight edge held vertical to the shelf, drag the powder across the shelf.  Where there are dark patches is an indication of depressions.  The area and depth can be seen from the spread of the visible powder and to some extent the density of the colour.

Making Shelf Flat

If you buy two of these large tiles, you can rub them together face to face in circular motions. The abrasion marks will show the high spots, with the low spots clear of those marks.  This will indicate the amount of work needed to get the whole surface even.  The smaller the unmarked areas, the less grinding will be required. You can add an abrasive with some water to form a slurry and continue to grind until everything is even. The use of water with the abrasives is important to eliminate dust which might be harmful, and to ease the grinding process.

The above is a manual process.  If you have a large enough flat lap, you can mechanise the flattening process.  Using decreasing grit sizes, you can grind the shelves level with a high degree of smoothness. You do not have to use a grit of less than 200, as the tile structure is even more coarse than that.

If you can't find unglazed floor tiles, you need to look at the back of the tiles.  Many floor tiles have a grid pattern on the back to ensure sufficient adhesive is used.  This makes getting the back, unglazed side flat more difficult or time consuming, because they will need to have the grid ground down to the lower surface.  In this case, it may be that you need to sandblast the glazed side before making sure it is flat.  The sandblasting can make a flat tile uneven by unequal times spend on various parts of the tile, so you have to check after sandblasting for the flatness.

Multiple Firings of Kiln Wash

Many people report that they fire multiple times on kiln wash that has not been renewed.  Most add coats over existing kiln wash.  They only remove all the kiln wash when it begins to crack, stick to the glass or gets divots.

We all know that kiln wash fired a second time to full fuse is likely to stick to the glass.  We also know that kiln wash fired to slumping temperatures lasts almost indefinitely.  Somewhere between the two temperatures the kiln wash undergoes a chemical change that makes it more likely to stick to the glass on the next full fuse firing. 

credit: Immerman Glass

Some people continue firing without adding additional layers of kiln wash until cracks, divots, or sticking occurs.  This leads to creating a fix after the failure of the kiln wash. This requires both finding a means of cleaning the kiln wash residue from the glass, and fixing the firing surface.

Others paint a layer of kiln wash on top of the existing separator before high temperature firings. This continues each firing with a fresh layer of kiln wash.  However, the same cracks, divots, and sticking occurs at some point, requiring a complete re-coating of the shelf, and getting the kiln wash off the glass.

credit: Sue McLeod Ceramics

Re-coating of a shelf takes a couple of minutes and can be done with simple tools.  A broad scraper will remove most of the kiln wash.  This can be followed by rubbing with an open weave sanding sheet as used for plaster board or other dry walling.  If you are worried about the dust – which has less risk than fibre papers – you can dampen the surface before beginning the cleaning process.

If the kiln wash has been on the shelf for many firings, it is more difficult to remove, requiring more effort than a single firing.  High temperature firings as for melts also make the kiln wash more difficult to remove. But the same process is used in these cases.

       

Kiln wash in firings at slump and low temperature tack fuses can be reused as many times as it remains smooth and undamaged since the temperature is not high enough to cause the chemical changes.

The ultimate benefit of renewing kiln wash is that not only less effort is required to clean and re-coat, than to fix pieces with kiln wash stuck to them, and also the cost of kiln wash is significantly less than fibre papers.

Removing kiln wash

Applying kiln wash

Kiln Maintenance

Switch off the kiln before doing any maintenance.

Before or after each use

Vacuum the inside of the kiln. Use a low suction setting, especially on fibre walls and ceilings. Stronger suction is possible when cleaning a brick floor.

Example of vacuuming around elements

Example of vacuuming lid without elements

An alternative to vacuuming the elements is to use the air compressor hose at low power to gently blow out any dust settled in the element grooves.  Do not do this for fibre insulated kilns, only brick.

Check on the kiln furniture – including shelves, boards, supports. Are they kiln washed and without scrapes, scratches, gaps? Has the kiln wash been fired to full fuse temperature? In both cases, clean the used kiln wash off the shelf and renew.

Check that the shelves and other kiln furniture are without cracks.

Clean kiln furniture of dust and debris.

Check the level of any item newly placed in the kiln - e.g., mould, or shelf replacement - with a spirit level.

Two examples of two-way spirit levels

 Check on the conditions and placement of the thermocouple.

Check on the elements.  Some may be sagging or hanging out of their channels.  Use tweezers to bring the coils closer together.  This shortens the length of the element and it then can be pushed back into the channel.  It may not have to be done after each firing, but checking will catch things before sagging becomes a major problem.

When the shelf paper is exhausted lift out the thicker papers and vacuum the shelf.  The Thinfire and Papyrus papers can be vacuumed directly or gently swept up and placed in a container for disposal.  Do not introduce any moisture to help reduce the dust.  This is not good for the kiln or you, as it could induce shorting out of the elements.

Monthly

Electrical parts: check the elements and their connections (normally at back or side).

First unplug or switch off the power to the kiln.

Check the screws on the connectors for the element tails are tight. Loose connections cause the wire to vibrate at the connection during the power phase. They heat up enough to melt the wire at the connection. For a single element kiln, it will simply lose power.  In multiple element kilns the remaining elements work much harder to achieve the temperature and provide uneven heating.

If the connectors are badly corroded , they need to be replaced.  This can be done without replacing the elements. Unscrew the connectors and put new ones on.  If the connector is fused to the element wire, you need to cut the wire as close to the connector as possible to maintain a length of wire for the new connector to be fixed.

Check the condition of leads and plugs supplying power to the kiln.  Make sure they are sound, not frayed and not kinked. Replace any frayed parts.  Take out any kinks in the power supply cable.

Any support pins or wires should be firmly seated in the brick work or supported by sound hangers.

Check the level of the kiln floor and internal shelves on a regular basis and every time the kiln and its internal furniture is moved.

Making a schedule of maintenance checks and noting on it the dates checked is a good idea for those who need reminders.

Formers

This post is not about the materials that go into the making of glass, but about ways of forming glass once melted or dripped into a space.

Formers are a bit different from moulds.  They are more like the formers used in concrete structures – they are there to resist the movement of the contained materials and give the form or shape desired rather than a natural flow.

These formers can be of anything that can resist the firing temperatures of the process.  Some of the materials are stainless steel, ceramics, fibre board and paper, vermiculite, kiln brick, and I am sure there are others.

Refractory Fibre

Most of these require a separator between themselves and the glass.  The ones which do not are untreated refractory fibre board and fibre paper. 

Most paper is not sufficiently strong to stand on its own. Instead it is used flat and the shape cut out of it.  It can be made in several layers and pinned together to achieve the height desired.  It should be lined in the interior with a thin fibre paper to avoid seeing the layers of the former in the edge of the glass.

For thicker work, fibre board can be used with the shape or form cut from it. Alternatively, it can be used on its side backed up by kiln brick or other material to resist movement. More information on methods and safety are here. 

If hardened, refractory board and paper will need separators between glass and former, just as most other materials will.

Sometimes the fibre board and fibre paper are not heavy enough to resist the flow of the glass.  You can use weights to help resist the movement.  At other times, the glass flows under the fibre and then you need something heavier.  Fortunately, there are a number of refractory materials that can be used.

Other common formers

Vermiculite board is another refractory material that can be cut and shaped much like fibre board.  The vermiculite needs to be covered with kiln wash where it might come into contact with glass or be lined with fibre paper or another separator.

Calcium silicate board can be used in much the same way.  It also needs a separator but does not stand up to such high temperatures as vermiculite.

Ceramics, especially in the form of cut up kiln shelves can be used as straight formers.  They have the advantage, over refractory fibre paper and boards, vermiculite and calcium silicate, of being heavy.  They can resist the movement of thick glass. They need to have a separator and usually a 3mm fibre paper, cut 3mm shorter than the final thickness of the piece, will provide the cushion in the movement that the glass needs.

Kiln brick is an often forgotten former.  The bricks can be cut and formed in many ways, even if not so freely as fibre board and paper.  The bricks do need fibre paper separators to keep the glass from getting into the pores of the brick.

Stainless steel is a common former too.  These are usually formed into an already determined shape and so are not so adaptable as many of the other formers.  Steel contracts much more than glass and needs a cushion of fibre paper, usually 3mm thick to avoid sticking to the glass.

More information on most of these formers can be found here.

Removing Kiln Wash from Shelves

There are at least three ways to remove kiln/batt wash from mullite kiln shelves.

One quick way is to use a broad wallpaper scraper held at a very acute angle to the shelf. This rapidly removes the separator. One down side to this method is that any uneven pressure can put a gouge into the surface of the shelf.

So a more gentle way to remove the wash is to use a drywall/plaster board sanding sheet or other open weave sanding material. This allows the powdered wash to come through the sanding material rather than clog up the material. The disadvantage to this is that it takes longer to remove the wash, although it does leave a very smooth shelf after many sandings.

A third way is to wash off the kiln wash. This is relatively quick, but it gets the shelf wet and requires a longer period before the shelf becomes dry. You can, of course put the next application of kiln wash on as soon as the shelf is clean. They both can dry off at the same time.


Power tools used to clean kiln wash from the shelves can induce low points in the shelf which will promote bubbles during fusing.  It is recommended to avoid power tools in removing kiln wash.

Glass on Drop Rings

When glass drops through a ring, you need to check on some things relating to the placement and firing.

When thinking about the relationship between the size of the flat glass and the size of the aperture, you need to remember how the glass behaves as it heats up toward the drop temperature.

Glass behaviour

The glass begins to sag at the middle of the aperture, however the glass is still relatively stiff.  The weight of the rim is not enough to keep it from rising from the ring. The rim of the disc maintains the angle from the centre of the drop to the edge, until it gets hot enough for the weight of the rim to allow the edge of the disc to settle back down onto the ring.  This is the source of a lot of the stretch marks at the shoulder of drops.

Rim width

To avoid the glass dropping through, you need to have an adequately sized rim.  The width of the rim sitting on the ring, needs to be related to the size of the hole.  

The consequence of an inadequate rim

I have found that for apertures up to 300mm diameter there needs to be at least 35mm on the rim.  The consequence of this is that your blank diameter needs to be 70mm more than the hole diameter.  For larger apertures – up to 500mm – you need 50mm, or 100mm added to the diameter of the hole.  I do not have the experience to say how much more is required for larger diameter drop rings.  There is more discussion on blank sizes here. 

Heat

The rate at which you heat the glass and the top temperature both have effects on the possible drop through.  

High temperatures. The higher temperature you perform the drop out, the more likely you will need larger rims or other devices to reduce the drop through possibilities.  It also promotes excessive thinning below the shoulder. 

Fast rates. The surface will become hotter than the bottom, but at different rates.  The glass over the hole is heating from both top and (to a lesser extent) bottom.  The rim is sitting on the ring and so heats only from the top.  The differential in heat may cause a break.

Weight. The thickness of the glass effects when the drop will begin.  The heavier the glass and larger the hole, the effective weight will be greater.  In these cases, you can use a lower temperature for the drop.

Additional methods.  You can use other methods to reduce the chance of a drop through.  Two of them are:

Weights. You can put kiln furniture on the glass rim to keep it from rising during the initial stages of the drop.  These must be placed symmetrically. Four or six pieces of kiln washed props or small dams would be sufficient up to 300mm diameter.  More would be required for larger apertures.  Of course, these will mark the rim, meaning that it must be cut off.

Inclined rings. Another possibility is to use an inclined ring, with the glass resting on the upward incline, so the glass is held above the aperture and is heating evenly until the drop begins.

First Kiln Selection

Glass fusing works best in top fired kilns.  Glass casting and some tall work are best with side or bottom elements too.  Compromises can be made of course.  The comparison of glass and ceramics kilns is important to understand.  

Kinds

Most of the following types of kilns are available for glass purposes.

Front loading.  These are good basic, multi-purpose kilns with good viewing properties.

Bell kiln.  This is where the whole of the heating chamber lifts up from the firing bed.  This is more common with very large kilns and is usually combined with lifting gear.

Clamshell kiln.  This is where the firing chamber is hinged, usually on the long side.  This kiln provides access from three sides. It can become too large to reach to the back of the kiln, so these tend to be rectangular.  The lid can also become too heavy for ease of movement and support.

Top loading.  Often called a coffin kiln, there are very good for casting or deep work, but are hard on your back while loading.  They need peep holes at appropriate levels to be able to monitor progress of the firing. These tend to have smaller floor areas than the clamshell.

Car kilns.  These are those where the firing chamber lifts like a bell kiln, but has the firing base on rails or tracks to move multiple firing bases under the firing chamber.

Modular kilns.  These are normally rounded kilns where each ring is controlled separately and can be placed on top of one another.  This is good for large heavy castings, as the refractory and glass reservoir can be placed on the base and the rings built up around the work.

All these kilns come in a variety of sizes.

Choose a kiln relevant to your current work.

The first thing you must decide is the kind and scale of work you intend to do in the near future.  It is too difficult to predict how your work might progress based on experiences with your current work.  It is better to by a smaller kiln that is ideal for the current work and then move to a different kiln, if necessary, or a kiln for different styles or scales of work.

The general advice is to buy as large a kiln as your budget and space and electrical installation will allow.  This remains the case with some precautions.  Think about how often you will fire - daily, a few times a week or a few times a month.   Think about how long it will take to fill the kiln.  A large kiln can take days or even a week to fill with small works. This would really limit the variety of things you could do in that period.  You would have to wait to slump until you had enough things fused to fill the space.  Indeed, you would need to have more moulds than if you had a smaller kiln.

I’m sure you can envisage a time when you will want to work larger than at present, but your first kiln will not become redundant.  It will continue to be useful throughout its long life.

Factors in the choice

Size. As already alluded, the size needs to fit with your current style and scale of work. 

Access.  How big a kiln can you get through the doorways?  How much bigger than actual external dimensions will the packaging make it upon delivery?  It is no use buying a kiln that must be taken apart, or all the packaging removed, to get it into your studio.  Of course, the wider the entrance(s) to your studio the easier it will be to get a larger kiln.  If you really need to have a large kiln, you might have to alter or move your studio space.  You also need to think about the kind of access to the studio.  Does the kiln have to come along the side of the house? Is the path paved or gravel? Stairs? Lift size? Parking for the delivery vehicle?

Space. The kiln also needs to fit into the space you have.  You will need about 15cm all around the outside dimensions for safety purposes.  This applies to ceramic kilns also, even though they routinely reach higher temperatures. The skin of the kiln does get hotter than is comfortable for your hand, but normally not hot enough to burn paper. You can reduce the front to back storage space by putting the kiln on wheels.  But the 15cm saved is not worth the time required to once again ensure that the kiln and shelves are level each time you move it. 

Accommodation also needs to consider access around the kiln to place work in the kiln, especially if you build elements in place on the shelf. 

Location within the studio is important, as the kiln needs to be near a power supply and in a place where it is away from the movement within the studio.

Power supply.  The nature of your power supply will also determine what size of kiln you should buy.  Note both the wattage and amperage required for the kiln and determine whether your household supply can cope with the energy requirements.  Usually a kiln can be run on household supply until it reaches the 1 metre² size, where three-phase power is required to have efficient use of the electricity.

Wattage. Kilns below the 1 metre² (approximately 1 square yard) in size have a need for at least 0.6 -1.2 watts per cm², or 4-8 watts per inch².  Once the kiln is larger, more power is required per area to accommodate the greater mass of the kiln.

Insulation.  All kilns require insulation.  This can be fibre or light weight brick, or a combination of the two.  These insulating bricks can be red hot internally, but only warm to the touch on the outside.  Generally, the refractory fibre – whether board or blanket – requires less energy to heat and cools more quickly in the critical devitrification range.  Most often the kiln floor will be made of brick to provide a firm base to support the kiln furniture.

Features

All kilns come with a range of features, many of them relevant to the size, but not all have the same ones, or the ones important to kiln forming.

Viewing ports.  These are variously called vents, ports, bung holes, etc.  Their importance is at least three-fold. 

·        These provide an opening(s) for you to view the progress of the firing, so you can add more time or heat, or skip to the next segment when adequate heat work has been completed earlier than expected.

·        They provide a means of venting the kiln.  This is important in the burn out of any fibre paper binders, and in allowing enough air to promote the oxidisation and maturation of the hot enamel colours.

·        These openings allow the kiln to safely cool more quickly at lower temperatures, say 300°C, but lower for thicker or more delicate pieces.

Opening.  The way the kiln opens is an important consideration.  Some kilns do not allow the kiln to be opened at all during firing.  This is not a desirable feature on a glass kiln.  It is important to have a switch that will turn the kiln off after a certain degree of opening, so that no contact can be made with a live element. 

·        A front opening kiln allows maximum flexibility to view the progress of slumps, drapes, tack and full fuse kilnforming.  It should have a switch to turn off the power to the elements after a certain degree of opening.

·        A top loading kiln allows you to add glass during a casting process, but is not suitable for working the glass during firings – E.g. combing, manipulation of a slump or drape.  This type of kiln occasionally has no allowance to open the top without turning off all the power to both the controller and the elements.  Avoid this, or have it changed.

·        A clamshell or bell kiln allows maximum accessibility during the loading phase and the forming stages.  Although a lot of heat is dumped forward, it is the easiest to use for combing and other manipulation of the glass during the firing. Again, this kiln needs a lid operated switch to cut the power to the elements when opened beyond a certain point.

Controller.  Although essential, controllers are often given as options, especially on smaller kilns.  There are at least two reasons for this.  There are a variety of controller styles and costs.  The buyer may already have their own controller, or wishes to specify the kind.  Controllers are significant costs involved in smaller kilns – sometimes being at least one-sixth of the price.  In general, the more features a controller has, the more it costs.

Controllers are often classified as “three-key”, or as full number pad.

·        The three-key controller – even if they have many more than three keys – is one where the numbers must be cycled through by holding an up or down arrow to change the numerical information.  This includes the programme number, segment number, time, rate, temperature, and sometimes other information. 

·        The full number pad controller will allow direct entry of numbers at each segment of the programming.  It will often have additional features, such as calculating the firing cost or kilowatts used, elapsed time, additional capacity for more saved programs, ability to control different areas of the kiln heating, etc.

Extras 

There are often things which will be worth considering purchase along with the kiln, but are not usually included in the base price.

Stands.  Smaller kilns range from table top - which do not need stands at all – through medium sized – which have optional stands – to larger ones that come with the stand integral to the whole kiln. Unless you intend to move your kiln about, it is not necessary to buy one of the metal stands. Even so, most of these stands come without wheels, so check that they do have wheels already attached.  If you will not be moving the kiln, you can use a wooden table with a refractory fibre board between the stub legs of the kiln and the table surface.  If the kiln does not have stub legs, you can set it on 4 house bricks. 

Kiln furniture. This consists of the refractory props and dams that will be needed in kilnforming.  The most essential are short (2.5cm) kiln posts to support the shelf.

Shelves.  Most shelves require a mullite/cordierite shelf to fire on.  This is a robust shelf that does not have the quartz/crystobalite inversions that ceramic shelves and tiles used for shelves have.  It is a good idea to buy one of these to fit your kiln at the time of purchase. Smaller kilns can use fibre board or vermiculite board as the shelf.  These can be purchased later.

Extractor fans.  These are available on many kilns. They are unnecessary on smaller kilns as they cool quickly anyway.  Larger kilns in a production environment may need quicker cooling, and these arrangements are very useful in those circumstances, but not others, as most kilns will cool in 8 – 16 hours without drawing air through the kiln.

There are a lot of other considerations in buying a kiln, but these are among the important ones, especially in selecting the first one.