Kiln Wash Removal

There are a variety of ways to remove kiln wash.  Many depend on whether the surface is flat, smooth curves, angles or textured.  Some are applicable to both.

Flat surfaces are the easiest to deal with.

Abrasive methods work well with a variety of tools. 

They can range from large paint scrapers to smaller ones with a Stanley blade inserted. 

Coarse open mesh plaster board (dry wall) sanding sheets are very useful. There are frames that you can fix them to, but sanding without the frame works well too.

Using power tools to sand the shelf is not advisable.  It is too easy to remove lots of material, including the surface of the shelf – even the hard, ceramic ones.  This leads to minor depressions in the shelf and consequent bubble difficulties when firing.

Do not be tempted to sandblast as that will, almost certainly, create small depressions in the surface of the shelf.  Sand blasting is only possible on steel moulds.

Wet

Wet methods are applicable if you are concerned about the dustiness of the process.  You can dampen the kiln wash on the shelf and sand or scrape as above.  You will create a paste or slurry in front of the scraper which can be bagged and put in the waste.

You can also use a lot of water and the green scrubby washing up pads.  Unless you use a lot of water, the kiln wash builds up in the scrubbing pads.

Some people use vinegar or chemicals such as lime away with the water. Both are acids – lime away being much the strongest.  I am sure these are used on the basis that kiln wash is based on lime.  However, the material that makes the kiln wash stick to the shelf is china clay which is barely affected by the chemicals.  In addition, the alumina hydrate is impervious to many chemicals available to kiln workers.

One drawback to using wet methods, is that the shelf is wetted and needs drying before use.  The amount of water used in applying kiln wash is minor in relation to washing or soaking the shelf to remove the kiln wash.

Do not be tempted to use pressure washers. Yes, they will remove the kiln wash, but also leave little divots in the shelf which will cause later problems.

Smooth curves

Kiln wash on moulds with smooth curves can be removed with flexible sand papers or the plaster board sanding screens.  Normally, the coating of kiln wash is thin and does not require a lot of pressure or effort.

It is possible to dampen the kiln wash and take it off with scrubbing pads.  Make sure you do not use excessive pressure.  If you have wetted your ceramic mould, you need to dry it very carefully, to avoid having the mould break in the next firing.  This is because trapped water can turn to steam and the pressure will break the ceramic. It is best to let the mould air dry for a week or so before putting it into the kiln to thoroughly dry at about 90°C for a couple of hours.

Do not be tempted to use a pressure washer or water pick, as both can erode the surface of a ceramic mould.

Curves with angles

Moulds with angled areas such as at the bottom or corners of a rectangular mould need a flexible abrasive to clean out the angles.  You can fold a piece of sand paper to use the folded edge to do the final cleaning out of the angles.

The same can be done wet, but all the precautions about wet removal of kiln wash need to be observed.

Textured

Textured moulds require much more care in cleaning the kiln wash away, to avoid damaging the images and textures.  The flat upper surfaces can be dealt with as though it was a flat kiln shelf.  The indentations need to be more carefully treated.  Folded pieces of sand paper can be used to clean the delicate areas.

To ease cleaning of textured moulds it seems best to use kiln washes without china clay as the binder.  These will brush out of the mould with a fibreglass bristled brush.  It is now popular to use boron nitride - often sold as Zyp - as a coating for these moulds.  This needs to be brushed out and renewed with each firing.

Removing kiln wash from glass

Kiln wash stuck to the glass can present greater problems, because you want to avoid marking the glass.  It is best to start with the least aggressive abrasive, such as a green scrubby, and progress toward more aggressive and abrasive methods.  When using the more aggressive methods, try the finest grit first to see if that will work, as it makes for less work cleaning up the grinding marks from the glass.

For flat glass, you can work with a succession of finer loose grits, or a succession of finer diamond hand pads.  

Flexible diamond impregnated sheets can be used for curved surfaces.  Again, this requires a succession of finer grits to get to the polished stage.

You can use small hand held rotary tools with diamond and felt pads to polish out stuck kiln wash.  This helps to remove some of the labour of polishing the glass.

Some people advocate the use of acids to remove the kiln wash.  However, you must remember that glass is an alkaline material and acids will tend to mark the glass.  Vinegar is a mild acid, but prolonged exposure will etch the glass.  Strong chemicals such as lime away or etching cream or hydrofluoric are all strong acids and will mark the glass after brief exposure to them.

Holes vs. Elevation of Moulds

Drilling holes and raising the mould are not the same. They achieve different things.  Drilling holes allows air out from between the mould and the glass.  

There are some things you need to check about the vent holes in moulds.

            Are the holes in the mould at last touchdown point(s)?

Sometimes the vent holes in moulds are made at convenient points rather than at the places where the glass will last touch the surface of the mould.  On a simple ball mould, a hole at the centre will be appropriate, as this is the last place the glass will touch. 

 On a bowl with a square base, the last places the glass will touch are the corners, so that is where the holes need to be.

The vent holes in this could also be at the other two angles in addition to those at the top and bottom of the picture.

Are there holes in the side of mould to allow air out from under the mould?

If there is one or more, there is no need to elevate the mould.  The air will move out from under the mould through the hole in the side. In general, moulds are not so uniform on their base that they fit the shelf enough to seal the displaced and expanding air underneath the mould. But you can be safe by elevating the mould on pieces of 1mm or 3mm fibre paper.

This mould has side vents, although the holes at the base may be a little large.

Are the holes clear?

This is more important.  If the vent holes are not open due to kiln wash or other things blocking the space, there will be no escape for the air.  The vents need to be checked on each firing to ensure they are open.

  

Does the mould need holes at all?

There are shallow slumpers and other simple moulds - such as a wave mould or any cylindrical mould form - that do not need vent holes, either because they are so shallow, or because the air can escape along the length of the mould.

More information can be found in this and related blog posts.

Large thick bubbles at the bottom of the glass

Not all large bubbles at the bottom are the result of the lack of holes.  Sometimes they are the results of too fast or too high a firing. Some notes on this are given in this blog entry. 

What does elevating the mould do?

The purpose of elevation is not allow air to escape from under the glass, although that may be a by-product.  Elevating the mould allows marginally more even cooling of the mould and glass if it is on a thick kiln shelf. It will not create any problems, but you need to be careful about how near the elements it will place the glass.  The elevation does not need to be more than 25mm, just as for the shelf above the floor of the kiln.

CoE of Paints for Fusing

CoE of the glass carrier for paints is a distraction.

Paint has been applied to glass and fired for at least seven centuries – long before CoE measurement.  The earliest enamels were intensely coloured glass powders applied to depressions in the base metal (iron, gold, copper, brass, etc) and heated.  More detailed images began to be created when the powers were mixed with a liquid binder and painted on either in a single, or multiple layers onto glass and metals.

Silver stain became popular in the 16^th century and has continued since.  This is a different way of colouring the glass, as the colour does not laminate with the surface, but is chemically combined with the glass.  Various silver salts produce different colours and vary in intensity at different temperatures.  This can provide a variety of effects at fusing temperatures where it “metalises”, providing ambers and blues.

CoE in relation to paint does not matter.

The amount of paint is miniscule in relation to the mass of glass to which it is applied, and so any incompatibility would not have sufficient strength to break the glass. If the paint’s glass carrier was too incompatible, it would come off instead of breaking the glass, in any case.

The composition of the fusing glass paints is largely unknown, although commonly supposed to be powdered glass frit. Some may be the same as enamels used in metal enamelling. Some others may be the same as the on-glaze ceramic colours. They all have glass as the carrier of the colour.  Still, the amounts of glass involved are very small and compatibility is not a concern.

In any case CoE is not the most important element in compatibility.

Slumping Glass that is not Tested Compatible

Is it Possible?

It is possible to slump unknown glass. This glass might be art glass, window glass, bottles, or any other glass whose characteristics are unknown by you.  There are some suggestions about the characteristics of some glasses in this post that can be used as a starting point.

Preparation of the Glass

Prepare the edges to their final finish before slumping.  This because the slumping temperature will not be enough to alter the finish of the edge significantly.  This preparation can be done with diamond hand pads, or wet and dry sandpapers.  Start with a relatively coarse grit. You may wish to do the initial shaping on your grinder. This will be between 80 and 100 grit.  Continuing with a 200 grit and working your way through 400 and then 600 grit will give you an edge that will become shiny during the slumping.

Cleaning

Clean thoroughly.  This is especially important when using glass that is not formulated for fusing.  Devitrification is more likely on these glasses.  Water with a drop of dishwashing liquid can be enough unless your water has high mineral content.  Then distilled water or a purpose made glass cleaner such as Bohle or Spartan should be substituted.  Finish with a polish to dry with clean paper towels. More here. 

Firing the Slump

Fire up slowly.  You should advance at about 100°C to 150°C per hour.  Set your top temperature around 630°C for a simple slump, for soda lime stained glass.  For bottle or window glass you will need a temperature closer to 720°C although the also are soda lime glasses.

It is best to start with simple curves, as there are fewer difficulties in determining what the glass is doing.  It will help you to learn the characteristics of the glass before you tackle the difficult stuff, such as compound curves or texture moulds.

Observation

It is necessary to observe the progress of the slump as you do not yet know the slumping temperature.  You want to know when the glass begins to deform so that you do not over fire.  Start watching the glass at about 10 minute intervals from about 580°C for stained glass and 680°C for window and bottle glass.  There is not much light in the kiln at these temperatures, so an external light is useful.  You can also observe the reflections of the elements on the glass.  When the image of the elements begins to curve, you know the glass is beginning to bend.

Altering the Schedule

Soak for at least 30 mins at the temperature when the glass begins to visibly drop. This may or may not be long enough.  Continue checking at 5-10 minute intervals to know when the slump is complete.  If the glass is completely slumped before the soak time is finished, advance to the next segment.  If not fully slumped, you need to extend the soak time. This means that you need to know how to alter your schedule in your controller while firing.  Consult your controller manual to learn how to do these things.

Stop the soak when complete and advance to the anneal. Continue the slumping soak if not complete after the 30 mins.  In some cases, you may need to also increase the temperature by 5-10°C.

Annealing

The annealing point will be about 40°C below the point that the glass visibly starts the slump. If you want a more accurate determination of the annealing point, this post gives information on how to conduct a test to give you both the slump temperature and the annealing point.  It also helps to determine the lower part of the tack fusing range (the lamination state), since it is not far above the slumping point that you will observe.

The annealing soak for a single layer, 3mm glass need not be long – 15 to 30 minutes.  The annealing cool can be as fast as 120°C down to 370°C.  For thicker glass and slumped bottle glass you will need a longer soak – 30 to 60 minutes – and a slower cool.  The annealing cool in this case could be about 60°C/hour to 370°C.  You can turn the kiln off at 370°C, if you wish, or keep the temperature controlled to about 50°C.  The rate for the final cooling can be approximately double the first cooling rate.  For a single layer of stained glass this could be 240°C, and for thicker glass about 120°C

Separator Cost Comparisons

Many people are concerned about the cost of kiln forming, but use fibre paper rather than kiln wash or powders, although it is many times more expensive. This may be a matter of convenience.  This leads me to an exercise in comparing relative costs and benefits of various separators.

Separators are essential to keep the glass from sticking to the shelf or mould that supports the glass. There are several forms of separators –

·         papers,

·         liquids and

·         powders.

Papers

The papers include the very smooth Thinfire and Papyros papers and the rougher papers ranging from 0.5mm to 6mm.  All these contain a binder of some kind. 

·         Papyros, Thinfire

·         Refractory fibre papers - .5 to 6mm

These are mainly suitable for flat surfaces.

Liquid

·         Kiln wash – there is a variety. Most have kaolin - china clay - as a binder.  A few do not.  These you can just brush off the shelf or mould after firing.

·         Colloidal Boron Nitride – a popular formulation is called Zyp.

These are suitable for both flat and curved surface applications.

Powder separators include:

·         Chalk

·         Talc

·         Alumina hydrate

These have applications directly onto the shelf or mould and onto refractory separators.  If used between glass sheets as in bending, very little is required.  This is similar when applied to existing refractory papers.  As a shelf bed, much more is required.

This analysis of separators shows the first choice is about the application, as some are not useful in a given situation.  But in all cases, there are choices in what separator to use.

I used a popular UK website to obtain comparative prices for the various materials.

Papers

Papyros paper is listed at £18.46. This is enough for 5 shelves at 52 cm sq.  The per shelf cost, assuming two uses per sheet, equals £1.85.

Thinfire is listed at £10.16. This is enough for 5 shelves at 52c m square. The per shelf cost, assuming one use, equals £2.03.

Liquids

400 g Bullseye kiln wash is listed at £3.96.  This enough for about 80 shelves at 52cm sq.  The per shelf cost equals £0.05.

400g of Primo primer is give as £6.06.  This also is enough for about 80 shelves at 52cm sq.  The per shelf cost equals £0.075 (i.e., 7 and a half pence).

Boron Nitride enough for about 25 shelves at 52cm square is listed at £63.93.  The per shelf cost equals £2.56.

Powders

25kg calcium carbonate is listed at £14.61. This is a one-use material.  Applied at half a centimetre thickness, it is enough for 700 shelves at 52cm square.  The per shelf cost is £0.02.

300gms talc is listed at £2.99.  this is enough for 8 shelves.  As this is a multi-use material, assume 10 uses.  This gives a per shelf cost of £0.035.

Alumina hydrate is listed at £9.99 for 500gms. Again, this is a multi-use material, so assume 10 uses.  This gives a per shelf cost of £0.04.

Ratios of cost between the least and most expensive (given the assumptions) is as follows:

·         Chalk =1

·         Talc = 1.75

·         Alumina Hydrate = 2

·         Bullseye shelf primer = 2.5

·         Primo shelf primer = 3.75

·         Papyros fibre paper = 92.5

·         Bullseye fibre paper = 101.5

·         Boron Nitride spray = 128

This illustrates that convenience most often wins over expense, as the boron nitride, Papyros and Thinfire seem to be the most popular separators.

High Temperature Wire for Screen Melts

You can use high temperature wire for screen melts. This is variously described as Kanthal or nichrome wire.  It is the same kind of wire used in the heating elements of your kiln.

wire with bent ends

To use the wire, you lay or weave the wire and support it on both ends.  Weaving the wire provides more support, but is not necessary, as the wire is strong enough to support a lot of glass.

first line of wires pushed into board

You need to have the wires as tight as the supporting material will allow. Straightening the wire before beginning to fix them will help, as will thicker wire.

The wires need support at each end, which can be brick, cut up shelves, or strips of tile.  If you do this, you can form a dam or vessel in which to put the glass without fear of it spreading over the edge.

I use fibre board for the support and just bend a right angle into each end of the wire to push into the board.  These can be arranged in any configuration, although for ease of illustration, I have used a rectangular arrangement of wires.

A grid of wires ready for kiln wash

Put the completed screen over a tray or sheet of plastic to collect the excess kiln wash.  Mix the kiln wash very thick and pour over the wires. I put the board with wires into the kiln to heat to about 200°C to help the wash stick.  I repeat a few times.

Make sure you coat the area surrounding the screen to avoid the glass sticking to the supports.

When the kiln wash has dried, knock off the stalactites of wash on the underside of the wire to prevent any excess kiln wash being incorporated into the final piece.

Place on kiln washed supports, and put the glass on top of the screen.

This is a relatively quick and inexpensive means of providing a custom shaped screen.  

One disadvantage of this over stainless steel rods, is that it is difficult to get enough kiln wash to stick to the wires to be able to pull them out easily.

Capping with Frit

Capping with a clear or tinted top layer is necessary in many cases of inclusions, or desirable when looking for depth or distortion in flat fused work.

Capping inherently has bubble creation potential.  The development of a bubble squeeze helps prevent the largest of bubbles.  It cannot eliminate all the trapped air that then turns into small bubbles around the inclusions or multiple pieces when covered by a sheet of glass.

An alternative is to do away with the sheet glass capping and instead use enough frit to provide the desired depth, or the necessary material to cover the inclusion.  In fusing with two large sheets, a fine covering of powder between the layers will help to eliminate bubbles.  However, this will not be enough to successfully cover metal or other inclusions, or provide the amount of glass to give an appearance of depth.

The size of frit to use in a given application can be determined from other styles of glass working. It is known from glass casting that the smaller the frit the greater number of small bubbles will appear in the fired piece.  This means that you need to use medium sized frit for cast work.  Fine frit is likely to produce many very small bubbles across the whole piece in fusing applications.  Large frit is likely to produce larger bubbles, as the pieces themselves trap air as they deform.  This means that medium frit is a good compromise between large and small bubbles in capping. 

The layer of frit should be at least 2mm thick.  This means a lot of frit is required to do the job.  To judge the amount, you can measure the area of a rectangle or circle in square centimetres and multiply that by 0.2 to give you the volume (in cubic centimetres) of frit required.  Multiplying the volume by 2.5 (the approximate specific gravity of soda lime glass) will give you the weight of frit needed to cover the area. 

Alternatively, if the piece is irregular, you can weigh the base and add the appropriate weight of frit on the top.  If the base is 2mm, no further work is required to determine the weight. Weigh the 2mm sheet and use the weight of frit to equal the base.  If the glass is 3mm, you need two thirds of the weight in frit, and so on for thicker glass.

Using frit to cap is unlikely to eliminate all bubbles, but it will reduce them to a minimum.

Steep Slumps

Not all steep slumps are deep.

An example of a deep, steep slump

An example of a soup bowl with steep sides

A square bowl with slightly less steep sides

A shallow plate or platter

Relative to the size, the above platter mould is a steep slump, although not deep. 

This can be slumped in two stages to obtain confirmation of the glass to the mould without distortion. 

One way to do this is to place powdered kiln wash in the mould so there is a gentle curve to the bottom. Place glass on the mould and do a slow, low temperature slump.

After first slump, empty the kiln wash back into your container (it can still be used as kiln wash). Fire again using the same slow low temperature schedule as for the first. 

It may also help to retain the rim on the shallow plate to cut your circle 12mm larger than the diameter of the mould.  This will allow a margin for the slight shrinking that even a low and slow temperature slump will cause.

Boron Nitride

What is boron nitride? What makes it a good separator?

Boron nitride is a heat resistant refractory compound of boron and nitrogen with the chemical formula BN. It is also chemically stable at elevated temperatures.  It exists in various crystalline forms that are similar to a structured carbon lattice. The hexagonal form corresponding to graphite is the most stable and soft among BN forms.  It is the form most useful in kiln forming as a smooth release separator, especially for steel.  It is also used as a high temperature lubricant, and has a wide use in cosmetic products.

There is a cubic form that is similar to diamond (called c-BN), but softer.  It has a superior thermal and chemical stability.  There is a harder form called wurtzite, but which is rare. Neither of these is of much use in kiln forming.

Hexagonal BN

Hexagonal BN (h-BN) is the most widely used form of boron nitride. It is a good lubricant at both low and high temperatures (up to 900C, even in an oxidizing atmosphere). Another advantage of h-BN over graphite is that its lubrication properties do not require water or gas trapped between the hexagonal sheet layers. So, h-BN lubricants can be used even in vacuum, e.g. in space applications. The lubricating properties of fine-grained h-BN are used in cosmetics, paints, dental cements, and pencil leads.  In kiln forming, the high temperature lubricating properties are made use of as separator between metal, ceramic and other supporting materials for the glass.

“Hexagonal BN was first used in cosmetics around 1940 in Japan. However, because of its high price, h-BN was soon abandoned for this application. Its use was revitalized in the late 1990s with the optimization h-BN production processes, and currently h-BN is used by nearly all leading producers of cosmetic products for foundations, make-up, eye shadows, blushers, kohl pencils, lipsticks and other skincare products.”   
https://en.wikipedia.org/wiki/Boron_nitride

It has wide application in materials to give them self-lubricating properties.  Boron nitride has the properties of stabilisation of materials, reducing expansion and resistance to electrical conduction, making for wide use in plastics and electronics among a wide variety of other products.

Health and Safety

There are some health issues related to its use.  It is reported to have a weak association with the formation of fibrous material in the lungs and so result in pneumoconiosis when inhaled in quantity in particulate form.  It is best to wear a dust mask when applying and to do it outdoors, as simple ventilation will not prevent dust settlement indoors.

Firing Schedules for Wissmach 96

Petra Kaiser is reporting that there are people finding cracks in white W96, which she cannot be replicate.  However, they are using strange firing schedules.

The most popular one appears as follows, in Celsius, with my comments.

166°C per hour to 232°C and hold 20

166°C is relatively slow. It is a rate I would use for a fused 6mm piece.  An unfired two-layer piece I would fire at 200°C to the bubble squeeze.  There is no effect in soaking for 20 minutes at this temperature.  If there is a worry (often expressed) that there will be thermal shock unless you let the glass catch up, slow the rate of advance to 134°C.  This is of course excessively slow for a two-layer piece. 

If, however, you are tack fusing onto two un-fused layers, then 166°C may be appropriate, as you are shading parts of the base from the heat of the kiln. But the soak is not necessary.  It does not do anything useful.

166°C per hour to 538°C and hold 20

As the rate for this segment is the same as for the first, I repeat the soak is not necessary.  If the glass survived the first 200°C at this rate, it will survive the next 300°C too. 

This rate for two layer pieces could be increased to 200°C without damage.

The 20-minute soak at this temperature again does nothing useful.  If the glass survived to this point, you can continue the temperature rise to the bubble squeeze at the same rate as in this segment.

278°C per hour to 621°C and hold 30

Although this rate is not excessive, there is no real reason to speed the temperature rise.  If you use 200°C from the outset to the bottom of the bubble squeeze, no time will be lost in getting to the bottom of the bubble squeeze.

However, this schedule leaves out the important second part of the bubble squeeze.  This is a slow rise to about 50°C above the start of the bubble squeeze process. 

Insert an advance of 50°C per hour to 670°C with a 30-minute soak

278°C per hour to 788°C and hold 15

788°C is a temperature given in the Wissmach tutorial on firing schedules.  However, Petra Kaiser has found that 771°C with a 10-minute soak is sufficient for a full fuse (or 765°C with a 12-minute soak).

The speed at which you reach the top temperature affects what you need to use as the top temperature.  This rate of less than 300°C will not require more than 771 as a top temperature. However a faster rate will require a higher temperature, and with it potential bubble problems, over firing, needling, and inconsistent results.

afap to 527°C and hold 120

This seems to come from the old Spectrum 96 schedules where a temperature equalisation soak was established above the annealing point.  Even if it were necessary, two hours is excessive.

The temperature equalisation of the glass should occur at the annealing point. Therefore, this segment is unnecessary.  And should be replaced by an AFAP to 510°C

55°C per hour to 510°C and hold 120

If the previous segment is eliminated, the rate in this one should be AFAP to 510°C with a soak of 30 minutes for a full flat fuse of 6mm.  There is no need for a longer temperature equalisation soak, as this is enough time for all the glass to be within 5°C of each part.

If you were tack fusing, a soak of an hour would be sufficient for a single layer of tack on a 6mm base.

28°C per hour to 399°C and hold 1

This rate is appropriate for a piece of 19mm.  A 6mm piece could use a rate of 80°C per hour.  A tack fused piece as described above could have an annealing cool of 60°C per hour.

Depending on the natural cooling rate of your kiln, it is possible to turn the kiln off at this point.  If you kiln cools off faster than the cooling rates given above, then you do need to programme a second stage cool.

  

55°C per hour to 93°C and hold 1

This is excessively slow for a 6mm thick full fused piece – a possible rate would be 200°C per hour.

The one-minute holds in these two down rates are only required where your kiln controller will not accept “0” as the number.  If the controller will accept 0, then use that, as 1 minute will not do much of anything, except confuse.

Writing and evaluating  schedules

When you are writing or looking at others’ schedules, review what is happening to the glass at various temperatures.  This excellent guide tells you what is happening to fusing glass at various temperature ranges.  Float glass has some different characteristics.

Combine that knowledge with what you are trying to achieve in the firing.

Guidelines on temperatures

Time versus Rate 

Assessing pre-programmed schedules

Making your own schedules