Overlaying Fibre Paper with Thinfire

Some people use a 1mm or 2 mm fibre paper with Thinfire or Papyros laid on top to get a smoother surface. It also allows assurance that the air can migrate from under the glass through the fibre paper. 

However, if the Thinfire or Papyros is laid down too many times, it fills the spaces between the fibres of the paper, and resists the passage of air.  This results in the large bubbles your are trying to avoid.  Experience will show how many times you can layer Thinfire without blocking the passage of air through the fibre paper.

This effect can also happen on sand beds.  The separating powders can build up and fill all the spaces between the sand particles. This resists the movement of air through the sand.  It leads to large bubbles just as compacted Thinfire on fibre paper does.

Is this practice of repeated layering of Thinfire and Papyros sensible?  The tipping point between achieving a smooth surface and creating bubbles cannot be known with certainty.  This means there is always a risk of bubble formation.

It is possible to use a vacuum sweeper with variable suction control  to remove the Thinfire or Papyros residue.  Using the lowest power and holding the end of the hose above the paper can lift the Thinfire or Papyros without affecting the underlying fibre paper.  Then a new sheet of Thinfire can be placed over the fibre paper to maintain the smooth surface.

Repeated layering of thin fibre papers can lead to a compaction that no longer allows the passage of air from under the glass during fusing. 

Tacking Freeze and Fuse to Base Glass

The question has been asked:

I'm wanting to add some freeze fuse pieces on to float and just fire to a tack fuse … in one firing instead of two …[to avoid] losing the detail on the freeze fuse pieces. The top temperature on freeze and fuse is 720°C versus a … float tack temperature of 787°C. [can this be done?]

My response:

What you are doing with the freeze and fuse process is sintering the glass particles together by holding at a low temperature for a very long time.  This binds the glass together without altering the overall shape of the object. 

Sintering

There is no reason why you cannot sinter the freeze and fuse piece on top of a base glass, if you pay attention to one major thing.  The freeze and fuse object will shade the heat from the base glass.  If you do not slow the rate of advance enough, you will break the base glass by creating too great a temperature differential between the part under the freeze and fuse piece and the uncovered part.

Another element to be considered, is that the frozen object is damp.  This will need to be dried by a slow ramp or it will further complicate the uneven heating problem.

Scheduling the Rate of Ramp

Choosing the rate of increase in temperature is determined by the dimensions of what is being sintered.  One widely practiced method is to double the total height and fire for that dimension.  For example, if the freeze and fuse is 8mm high, add that to the 6mm base and fire for 28mm – (6+8=14)*2 =28mm.

Another slightly less cautious approach is to multiply the total height by 1.5 and use the firing conditions for that thickness.

Determining the rate of advance for the thickness you have calculated – by either method - can be aided by using the Bullseye chart for annealing thick glass.  Look at the final cooling rate in the chart for the nearest thickness. In this case, use the one for 25mm.  The cooling rate is given as 90°C per hour.  If the glass can safely cool at that rate, it should also survive that speed of heating at the start.

If you chose the 1.5 factor, the thickness to schedule for will be 21mm.  This is between the 19mm and 25mm thicknesses given in the Bullseye chart.  The cooling rate given for 19mm is 150°C and and for 25 is 90C. As 21mm is almost the mid point between the two, you can halve the difference in rates (150 and 90) to give 120°C as the rate of advance. Although in both schedules using these rates of advance for the described circumstance, I would add a soak at 250°C for 20 minutes, to be cautious.

Remaining Parts of the Schedule

Sintering Soak

The length of soak for the sintering stage can be the same as the soak for the freeze and fuse, as you will be both sintering the glass pieces together and to the base glass too.

Anneal Cool

The annealing soak and cool should follow the rates given for the calculated thickness - in this case for 21mm or 28mm.

The Bullseye chart Annealing Thick Slabs can be used for all types of soda glass (which includes float glass) to determine the soak times and cooling rates.  You only need to make alterations for the annealing temperatures.  The annealing temperature I use for float glass is 540°C. 

The first two stages of cooling are 55C each, so simple subtraction from the annealing soak will give the temperatures for each stage of the cooling. If we use the calculated 21mm thickness, the soak time will be 3.5 hours at 540°C.  Then the Bullseye chart's displayed cooling rate of 20°C will apply from 540°C to 485°C, and the cooling rate of 36°C will apply from 485°C to 430°C. The final cooling rate of 120°C will be from 430°C to room temperature.  The chart for these adaptations is described in the post about adapting the Bullseye chart for annealing.  The reasons behind these operations are given in the ebook Low Temperature Kilnforming.

Hake brush

Bamboo handle hake brush

The hake (pronounced hah–kay) brush was developed in the far east.  It has several variations – the original consisted of a group of bamboo brushes bound together in a line.  These are still made and used. Many modern hake brushes have a broad wooden handle with a wide line of hairs.  These brushes are made of very fine, soft hairs - often goat hair is used. 

Flat wooden hake brushes

The flat hake brushes are most often cheaper and in a wider variety of sizes than the bamboo ones.  I prefer the bamboo for the feel in the hand that the broad handle gives.  With the longer hairs, it holds more moisture and delivers even amounts of kiln wash even with long strokes. 

Use

These brushes can hold a lot of moisture and deliver it evenly.  This makes it good for laying  down large areas of even colour in watercolours, and in glass painting. The same characteristic makes it very good for coating shelves with kiln wash.  The brush should be filled liberally with the paint or kiln wash. The brush should be gently shaken to remove any excess. Hold the brush nearly vertical and let the bristles barely touch the surface as you move along in smooth sweeps across the surface.  This allows the kiln wash to be evenly spread with very few brush marks.

Maintenance

One drawback of these brush is that the fine soft hairs are difficult to bind into the ferrule.  This results in the brushes often shedding hairs onto the shelf as it is being coated. A tip I learned from Bullseye is to treat the new hake brush with superglue at the base of the hairs. It does not have to be super glue.  It can be any runny glue, or diluted PVA.  I prefer super glue, even though it is reported to have some sensitivity to moisture. You can work the glue into the centre by using a needle to poke at the hairs to move the glue toward the centre of the bristles.  The glue binds the hairs in addition to the binding at the ferrule, and so keeps the brush from shedding. 

I did this on my bamboo handle hake brush a couple of years ago and it is not yet shedding hairs during applications of kiln wash.

Make sure you clean the bristles immediately after using to avoid any material drying among the hairs and causing them to break when next used.  To clean the brush, you only need running water run through the bristles.  Do not scrub the bristles against anything.  The hairs are delicate.  Set the brush aside horizontally to allow water to drip off and the hairs to dry.  Setting the brush upside down when wet allows water into the bindings of the hairs.  Putting it with the hairs down onto a surface deforms the hairs, making it difficult to straighten them later.

A hake brush is among the most useful tools to put kiln wash onto shelves and moulds because it holds so much moisture.  It does require maintenance to ensure the hairs do not shed and that the delicate hairs are not broken.

Needle Points

Often fused glass has prickles or needle points around the edges and especially at corners after firing.

This illustration is from Glass Fusing Made Easy

The nature of glass and its interaction with the separators is the cause.  As you heat glass it expands. Once the cooling starts, the glass contracts. Often a particle of the glass sticks to the separator while the rest continues to contract. This dragging of the glass along the separator results in the creation of little sharp points developing as the glass retreats to its final dimensions.

The best solution I have found to reducing the points at corners is to blunt any points or corners before assembly. Only a tiny amount of glass needs to be removed from the corners to reduce the possibility of these points being developed.

Small needle points can also develop along the sides of the glass too.  These are more difficult to avoid.  The most successful method for me is to use a loose separator.  This can be Thinfire, Papyros or a fine dusting of alumina hydrate or powdered kiln wash.  Although less widely available, talc can be used. Talc is known to be carcinogenetic with high exposure, so breathing protection is needed. All these powders provide enough lubrication to allow the runny glass to slide without sticking. 

Of course, you can use boron nitride, which is very slippery, but the cost of it makes it expensive in comparison to the other methods, including using fine diamond pads to remove the needles.

An additional consideration is the temperature you use.  The higher the temperature, the more the expansion.  Expansion rates are almost exponential above the brittle phase of the glass.  Reducing the temperature by 20C and doubling time or more means the glass does not expand so much and the additional time allows the desired profile to be achieved.  

Of course, paying attention to volume control - using 6mm or more thickness - will help to reduce the needle points.  A 3mm sheet both expands and becomes thicker at the edges by drawing more glass from the interior and the edge while attempting to reach 6mm.  This means there is an increase in the needling effect.  Although a 6mm piece retreats on cooling, it does not have the additional thickening effect of a 3mm piece.  Even a 9mm piece retreats on cooling, although the final piece has a larger area than at the start. 

- - - -

There are various preventive measures that can be taken to avoid needle points on fused glass.  These range from altering the edges of the glass, using fibre papers that turn to powder, using refractory powders, or boron nitride. Post firing solutions relate to cold working.

Vitrigraph Pot Liners

Stainless steel vitrigraph pots are durable replacements for ceramic pots that do not last many -  if more than one - firings. But cleaning is not straight forward. Most recommendations seem to concentrate on cleaning by banging the metal to break the glass away from the sides and bottom.  This seems more brutal and noisier than necessary. It will eventually dent the metal and possibly become unusable.

An unlined ceramic pot

An alternative is to line the sides with 1mm or 2mm fibre paper.  Paper of this thickness has enough fibres that the paper will stick together and not contaminate the pull.  It will still protect the metal from glass sticking At the conclusion of the firing and after the cool down, you can remove the fibre paper and have clean sides.

Instead of placing the glass in a bare pot, you can  line the pot with fibre paper 

 

It is possible to put a piece of fibre paper on bottom with a hole in it to match the pot’s hole.  There is a slight risk of drawing fibres into the pull, although I have not experienced it yet.

This method also works with ceramic pots.

Viscosity of Colours

“I have been advised in the past, that blue fires quicker. I was told this by a Master glass maker.”

Viscosity has some relation to colour and intensity.  But you should note black & stiff black are both of the same intensity, and are fusing compatible, but have different viscosities.  This shows that colour is not the only determinant of viscosity, as the stiff black shows the viscosity can be adjusted within the same colour.  The quotation above indicates that the reasons behind any declarative statements need to be investigated.

Some factors in viscosity

Opalescent colours tend to be more viscous than their transparent counterparts.

It is the metals that develop the colours that produce much of the difference in viscosity.  The same metal can produce different colours in different furnace conditions, so viscosity cannot be assumed to be directly related to colour. 

Some people in the past have done their own tests on viscosity and colour relationships, but I have no access to them.  More recently Bob Leatherbarrow shows (Firing Schedules for Kilnformed Glass, 2018, chapter 7.2.5, p.88) some slumping tests done with opalescent glass. It shows how much less viscous black is than white, and that white is the most viscous.  The other results show red a little less viscous than white, then some greens, yellows and oranges, other greens, purple, pinks (in that order) and of course, the least viscous is black.

Transparent glasses tend to be less viscous than opalescent glasses.

How does this information relate to kilnforming practices?  It indicates that a piece with the less viscous glasses requires lower temperatures or less heat work to complete the forming of the glass than more viscous glasses.

When you have a combination of more and less viscous glasses in a piece you need to fire more slowly to ensure all the glass is thoroughly heated through and will deform equally.  You will need to observe and be prepared to move the piece on the mould to straighten it up.

Do your own viscosity tests

You can do your own tests for viscosity differences by arranging 10mm wide strips all the same length (about 30cm) of different colours. These should be placed on a kiln washed pair of narrow batts set parallel to each other 25cm apart and about 15cm high.  Fire at about 150°C per hour to about 650°C, setting the soak to 30 minutes.  Observe at intervals from 620°C.  Stop the firing when the least viscous has almost touched the floor of the kiln. When fired all together at the same time you can see the relative viscosity of the colours tested.  You can label these and store them, or tack fuse these labelled curves to a piece of base glass for future reference.

Achieving the Striking Colour

"Is there anything special I have to do to fire striker glass?  Can I mix striker and non-striker in the same kiln or piece?"

Strikers generally need a two-hour soak at slumping temperatures, about 660C.  This heat soak helps ensure full development of the colour. If the soak is not long enough, the colour may not achieve the target colour at all, or be paler than anticipated.

The rate of advance to the heat soak is not critical.  But it does need to be the appropriate rate for the thickness and nature of the assembly of glass being fired.

If you were to have too short a heat soak, you can fire again to help mature underdeveloped colours.  This will, of course, change the profile of the finished piece.

Strikers are compatible within their manufacturer’s own range. So, they can be combined in the same piece as any other of the glass in the fusing compatible range.  That means that they can be fired in the same kiln load as non strikers.

The two-hour soak at slumping temperature will not harm the later stages of firing, but it might lead to use of a slightly lower temperature tack fusing than without the long heat soak.  That is because of the heat work put into the glass at the lower temperature.   Only observation will tell you how much less temperature is required.  It may be possible that only a little less time at the forming temperature is required.  Again, only observation will tell you that.

Strikers require a heat soak to mature the final colour.  These striking glasses are compatible with the rest of the fusing range from a single manufacturer. Glass from different manufacturers must be tested for compatibility before combined into a project.

More Information on Citric acid

Vinegar is not recommended for cleaning of glass, and especially not to soak glass in to remove kiln wash or investment materials.

The reasons for avoiding vinegar are that

·        Dilute vinegar - as culinary vinegar is - attacks glass, giving a mild etch to the surface similar to devitrification.  Concentrated vinegar – oddly - does not attack glass as strongly.

·        It is of variable quality – due to uncontrolled strength, various culinary additives, etc.,

·        It has a strong odour, and

·        It takes a long time to work.

Citric Acid

However, there is an acid which works very well to remove investment materials and kiln wash without affecting the glass.  It is the humble citric acid.

Citric acid is a weak organic acid that has the chemical formula C₆H₈O₇. It occurs naturally in citrus fruits, although that is not the best source for cleaning purposes.

More than two million tons of citric acid are manufactured every year. It is used widely as anacidifer, as a flavouring agent and chelating agent. It is the last that is of most interest to kilnformers.

Chemical characteristics

A citrate is a derivative of citric acid. There are many formulations. Two examples are a salt that is named trisodium citrate (also known as sodium citrate); and an ester called triethyl citrate. We are more interested in the first as it is cheap and widely available.

The citrate ion forms complexes with metallic cations.  It forms complexes even with alkali metal cations. This makes citric acid an excellent chelating agent, especially of interest in removing kiln wash and refractory materials from glass.

Chelation 

This is a type of bonding of ions and molecules to metal ions. The agents are usually organic compounds. Chelation is useful in applications such as providing nutritional supplements, in chelation therapy to remove toxic metals from the body, in MRI scanning, in chemical water treatment to assist in the removal of metals, and in fertilisers, among other things. 

Citric Acid as a Cleaning and chelating agent

Citric acid is an excellent chelating agent, binding metals by making them soluble. Among many cleaning uses are:

    to remove and discourage the build-up of lime scale, from boilers and evaporators. 

    to treat water by chelating the metals in hard water, cleaners produce foam and work better without need for water softening. Citric acid is the active ingredient in some bathroom and kitchen cleaning solutions. 

    A solution with a 6% concentration of citric acid will remove hard water stains from glass without scrubbing. 

    Citric acid can be used in shampoo to wash out wax and colouring from the hair. 

    In industry, it is used to dissolve rust from steel and to form a coating on stainless steels to resist corrosion.

Its use in kiln forming is to make use of the chelation properties when dealing with kiln wash and investment material residues.  Aluminium hydrate is the main ingredient of all kiln washes.  When it becomes bound to glass, it is impervious to almost all chemicals.  The chelating property of citric acid enables the bond between the glass and the kiln wash to be broken by incorporating the molecules within its own, making a colloidal solution.  This process is approximately 6 times faster than any vinegar solution and without the odour and etching risks.

A sample of the affected glass followed by 4 hours in citric acid and 24 hours in vinegar.
Credit: Christopher Jeffree

A 5% solution made up with 50gm of granular citric acid in 1 litre of water is all the strength that is required. The affected glass can be soaked in this solution for the time required to complete the chelation without the risk of etching, and without needing ventilation to remove smells.  Unless you are using a lot of cleaner, it is better to make up much smaller amounts as mould can grow on this organic solution.

A 5% solution made up of 50gms citric acid in 500ml of water and 500ml of isopropyl alcohol makes an inexpensive and effective glass cleaner. However, if left for a length of time, it becomes sticky.  Apply the solution, scrub the glass and immediately wipe off the solution.  Then polish the glass dry. The alcohol in the solution makes keeping large quantites possible. 

This post was compiled with the assistance of Wikipedia, Christopher Jeffree and my own experience.

Although this post remains valid, there is another chemical for long soaks to remove mould material or kilnwash.

Placing of glues

The placing of glues to hold the glass pieces temporarily is important.  Often unsightly black marks appear due to inappropriately placed glues.  Bubbles can form between layers  and even appear to come from underneath the glass for the same reason.  Placing is often more important than the amount of glue used.  Still, the amount used should be the minimum to hold the glass from moving from bench to kiln.

Place glues at edges of the pieces to be secured during movement.  This allows the burn-off of the glue to evaporate without being trapped under the glass.  If you use very runny or diluted glues, the capillary action will draw the required amount of glue under the glass piece to form a secure adhesion.

Glues burn off and leave the glass pieces unsecured long before the glass becomes tacky enough to stick together.  This means that if your stack of glass will not stay in place without glue as you build it, the glass will collapse or move in the kiln.  Glues are only suitable to stabilise the glass pieces while moving to the kiln.

Two recommended glues that burn off cleanly are the Bullseye Glasstac (more fluid) and the Glasstac gel (more viscous)

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.

Bubbles on Drop-out Rims

Sometimes people doing dropouts get bubbles or unevenness on the rims of their pieces.  This means that it is not suitable to leave the rim on the piece.  Most times, this does not matter, as you intend to cut the rim away. But if you do want to have a rim these uneven surfaces are unsightly and not suitable for high quality pieces.

One person has indicated that they used a schedule of 250°C per hour to 520°C for a 30-minute soak and then proceeded at 330°C to the top temperature of 710°C.  This is probably too fast a heat up at the second segment.  Slower rates of advance are advisable.

One of the advantageous methods of scheduling for dropouts is to put the heat into the glass steadily.  I suggest there are two problems with the rates of advance and soaks in the above (partial) schedule.

The soak at 520°C would be more useful if it were at around 600°C.  This would allow the heat to be distributed throughout the glass before it begins to droop significantly.

The rapid advance of 330°C is much faster than needed, or desirable.  This rapid rate of advance allows the glass to move into the aperture, before the rim is plastic enough to stay on the supporting ring.

These rough drawings show how the rim initially rises from the ring, pivoting on the edge of the aperture.  This happens on all moulds (drops or slumps) where there is a rim.

With a rapid rise in temperature the raised rim edge gets more heat than the depressed middle, as it is closer to the elements.  This additional heat allows the edge of the rim to curve downwards forming air pockets as the edge touches back to the supporting ring.

Some people use fibre paper between the ring and the glass to prevent bubbles. This addition allows a passage of air from under the glass and reduces bubble formation.

Others have developed sloped drop out rings that eliminate the rising of the glass from a flat ring.  The glass is suspended above the aperture and only touches the edge of it as the glass softens. These crude drawings show the process.

To be certain of avoiding air bubbles under the rim of dropouts whatever the style of ring, you should use moderate rates of advance, with a possible soak at around 600°C which is before the glass begins any significant movement. The moderate rate of heating should be continued after this soak, rather than increased.

Drying kiln wash

“Dry your kiln wash between coats and before firing.” 

This is a frequent statement when talking about renewing kiln wash on shelves and moulds.  The main reason given seems to be that there will be less risk of creating bubbles by evaporating moisture.  The air drying will reduce moisture in kiln is a second reason.

There are some difficulties with this statement and reasons.

Drying between coats of kiln wash means you are applying liquid over powder. This can promote clumping and streaking through a too rapid absorption of water by the dry kiln wash. Also, it makes applying kiln wash a lengthy process.  It is not like painting a door or even a floor, where you must allow drying to avoid streaks. 

Credit: Ceramicartsnetwork.org

Applying kiln wash by brushing is smoothest if all coats are done at once.  This is what happens if you spray kiln wash on your shelves and that gives a smooth surface.  If it were otherwise, drying between coats would apply to spraying too.  Drying between coats promotes streaks in the applied kiln wash that needs to be smoothed before use.  This of course, does need to be done after the kiln wash has dried.

Drying before using the shelf or mould is unnecessary. The evidence I have to offer is that I frequently fire within an hour of applying fresh kiln wash to a cleaned shelf. I have had no problems with creating bubbles or glass picking up the kiln wash. The shelf dries, with a moderate rate of advance, long before the glass settles into the texture of the surface.  It is only as the glass settles into the contours of the kiln wash that it seals air, or any other material, under the glass.

The pigment in most kiln washes is to tell you which shelves have not yet been used.  If they are fired dry at even moderate temperatures, the pigment disappears.  Then you have removed that indicator of freshly prepared shelves or moulds.

Drying of kiln wash before use in not necessary.  If you wish to be cautious, air drying will be enough to avoid any problems with moisture.

What are enamels?

Not all enamels are equal

Enamel paints

This description refers to a paint that air dries (or with minimal heat) to a hard finish (usually gloss). Most commercially available enamel paints are significantly softer than either vitreous enamel or heat cured synthetic resins. The term "enamel paint" generally is used to describe oil-based covering products, usually with a significant gloss finish. Many latex or water-based paints have adopted the term.

Enamel paint has come to mean a "hard surfaced paint" and usually is in reference to paint brands of higher quality, floor coatings of a high gloss finish. Most enamel paints are alkyd resin based. Some enamel paints have been made by adding varnish to oil-based paint. Enamels paint can also refer to nitro-cellulose based paints. Nitro-cellulose enamels are also commonly known as modern lacquers.  These have been largely replaced by synthetic coatings like alkyd, acrylic and vinyl.

More information at: https://en.wikipedia.org/wiki/Enamel_paint

Enamel paints are used for coating surfaces that are outdoors or otherwise subject to hard wear, or variations in temperature.  A widespread application is in paints for cars. It is also used frequently to decorate or label bottles due to the low curing temperatures of some formulations.

Vitreous enamels 

Vitreous enamels are used in a variety of circumstances.  Metal signs are most frequently enamel coated; they are used in ceramics as over glazes;  and they are used on glass in many circumstances.

Vitreous Enamel is simply a thin layer of glass fused at high temperature on to the surface of a metal or glass. Vitreous Enamel can be defined as a material which is a vitreous solid obtained by smelting or fritting a mixture of inorganic materials.  The word enamel comes from the High German word ‘smelzan’ and from the Old French ‘esmail’.

The key ingredient of vitreous enamel is finely ground glass frit. Colour in enamel is obtained by the addition of various minerals and metal oxides. 

Vitreous enamel is made by smelting naturally occurring minerals, such as sand, feldspar, borax, soda ash, and sodium fluoride at temperatures between 1200°C and 1350°C  until all the raw materials have dissolved. The molten glass which is formed is either quenched into water or through water-cooled rollers. This rapid cooling prevents crystallisation. The resulting frit is ground in a rotating ball mill either to produce a water-based slurry or a powder.

At the milling stage, other minerals are added to give the properties and colour required of the final enamel. Different enamel colours can be mixed to make a new colour, in the manner of paint. Enamel can be transparent, opaque or opalescent.

More information at: 

https://www.vea.org.uk/what-is-enamel/

https://en.wikipedia.org/wiki/Vitreous_enamel

https://www.iom3.org/vitreous-enamellers-society/vitreous-enamel-information

Metal enamelling

Modern frit for enamelling steel is typically an alkali borosilicate glass with a thermal expansion and glass temperature suitable for coating steel and other metals. Raw materials are smelted together between 1,150 and 1,450°C (2,100 and 2,650°F) into a liquid glass that is directed out of the furnace and thermal shocked with either water or steel rollers into frit. Vitreous enamel is often applied as a powder or paste and then fired at high temperature. This process gives vitreous enamel its unique combination of properties. The smooth glass-like surface is hard; it is scratch, chemical and fire resistant. It is easy to clean and hygienic.  It all started 3500 years ago in Cyprus. Since 1500 BC, enamelling has been a durable, attractive and reliable material.

More information at: 

https://en.wikipedia.org/wiki/Vitreous_enamel

https://www.vea.org.uk/what-is-enamel/ 

Enamels in Ceramics

Overglaze decoration, overglaze enamelling or on-glaze decoration are all names for the method of decorating pottery, where the coloured decoration is applied on top of the already fired and glazed surface, and then fixed in a second firing at a relatively low temperature.  The colours fuse on to the glaze, so the decoration becomes durable. This decorative firing is usually done at a lower temperature which allows for a varied and vivid palette of colours, using pigments which will not colour correctly at the high temperature necessary to fire the clay body.

More information at:  https://en.wikipedia.org/wiki/Overglaze_decoration

Glass Enamels

Glass enamels are produced in the same way as enamels for metals and ceramics.  The frit characteristics are adjusted for various applications and temperatures.  This combination of finely ground frit and metals for colouring are often combined with a binder or carrier medium.  It is similar to vitreous enamel on metal surfaces, but the supporting surface is glass. It is also close to "enamelled" overglaze decoration on pottery, especially on porcelain, and it is thought likely that the technique passed from metal to glass (probably in the Islamic world), and then in the Renaissance from glass to pottery (perhaps in Bohemia or Germany). 

Glass may be enamelled by sprinkling a loose powder on a flat surface, painting or printing a slurry, or painting or stamping a binder and then sprinkling it with powder, which will adhere.  The powdered frit can be in the ceramic on-glaze composition suitable for fusing or casting temperatures, or it can be adjusted for slumping temperatures as in the traditional glass stainers’ enamels. It can produce brilliant and long-lasting colours, and be transparent, translucent or opaque. Generally, the desired colours only appear when the piece is fired, adding to the artist's difficulties.

More information at:  https://en.wikipedia.org/wiki/Enamelled_glass

The term enamel is applied to a wide variety of coating materials.  The range of usage is indicated, and the manufacture and applications of vitreous enamels is indicated.  The term enamel is not properly applied to finely ground coloured glass in a medium.