Wednesday, 3 May 2017

Channels in Jewellery Items

The principle in forming channels in fused glass is to keep the space open with something that will survive the firing and can be easily removed.

You can use kiln washed wire, mandrels, or tooth picks which you can pull out after cooling. These tend to leave a residue of the kiln wash behind. So this is best used on opaque items.

You can use rolled or cut fibre paper, which can be washed out after cooling, leaving a clean hole. This is works well on transparent items.

Both these methods tend to leave bumps over the channel. So you can make a three layer piece. Cut the middle layer short enough to allow the element to keep the hole open (toothpick, cut piece of fibre paper, wire etc.) to be placed with enough overlap of the top layer to catch the bottom layer. In this kind of setup you need to make the top layer a bit longer than the bottom layer. Make sure you are generous in the length of the "hole keeper" so if the glass (now possibly 9mm) does expand you do not trap the material inside.

Of course on a three layer set up like this you could use thin glass which would give you about 6mm of thickness thus eliminating the spread due to volume. In this case you would need to use fibre paper or wire that is about 1.5mm high/thick. It is probably best to have a thin piece of glass on each side of the “hole keeper” to ensure the glass does not retreat due to lack of volume.

You can experiment with a layer of standard and two of thin in various combinations to find the one you like best.

Wednesday, 26 April 2017

Borax solutions

A borax solution can act as a devitrification spray. That is its usual application in kiln forming.  But it can be used in other ways too.

Borax is a flux helping to reduce the firing temperature of glass. So, it can be used as a medium for powdered mica which can be painted or sprayed onto the glass. It also helps reduce the oxidisation of included metals.

Obtain borax that has no additives. Put a couple of teaspoons into water and bring to a simmer. Remove from the heat and cool. Decant the almost clear liquid off the sediment and you have a saturated solution of borax ready to use. 

If you are really parsimonious, you can add water to the crystals remaining in the pot and heat to get another saturated solution. You could do this until there was no residue, but that would get tedious.

Add a couple of drops of washing up liquid to the solution. This is enough to break the solution's surface tension. It helps to give an even distribution of the solution across the clean glass by reducing the beading of the liquid that otherwise occurs.

You can paint the solution onto the material - glass or metal - with a soft brush such as a hake brush, or you can spray it on with a pump spray container.  Be careful to clean the spray container immediately, as borax crystals form quickly.

Wednesday, 19 April 2017

Light and Dark in Designs

Chiaroscuro – This word borrowed from Italian ("light and shade" or "dark") refers to the modelling of volume by boldly contrasting light and shade. 

Glass artists need to be very cognisant of light and dark, both in terms of colour selection and in terms of density. A very thick dense glass of a dark shade of any colour will create a much more intense darkness than glass that is thinner and less dense.

In terms of colour, lighter hues go where the sun shines or where the eye is to be drawn. Pastel shades indicate brightness and light. Within some opalescent and art glasses it is possible to find a shade of colour graduating to white or light yellow. 

Shading can be achieved by using the white areas to indicate where light is falling. A denser dark glass can be used to indicate where light does not fall, or where very little light can filter through. It can also play the part of negative space.

Sometimes, it is useful to use a monochrome scheme to assist in determining where the light and dark should be, as in this pear:

The contrast between light and dark can be used in several ways. Darkness can indicate depth of field or distance when used in a general landscape. Or, it can be used to bring a foreground out, making other elements more vivid.

The key thing to remember in using stained glass is to not be afraid of dark glasses. They can very useful, even if of very odd hues of colour.

Wednesday, 12 April 2017

Firing wire inclusions

Wire and other metal inclusions often cause bubbles to occur around them.  The standard solutions are to add frit to the corners, or powder or fine frit around the inclusions.   You can also flatten the wire or metal to reduce it height. These most often work well.  Sometimes though they don’t eliminate big bubbles around the metals.

In this case think about firing upside down. This is not the whole piece; it is only the inclusion and the bottom layer of glass.  Place the wire or other inclusion on the prepared shelf. It will be most successful if placed on 1mm or thicker fibre paper to allow any trapped air to escape through the fibre.  Place the base glass on top and take to a tack fuse with a bubble squeeze included.  You might even want to consider cutting the base larger than the final piece to be able to cut off the thickened edges and make a more successful piece at the end.

After tack fusing upside down, the inclusion will be imbedded in the glass with an almost flat surface and little in the way of air pockets at the edges.  Clean very well, especially any spalling from the metal and of course, clean the glass thoroughly.  Cap and fuse with a bubble squeeze again.  The bubbles around the inclusion should be minimal if not eliminated.

This method will allow the glass to sink around the glass making a much flatter piece for the capped full fuse. It should also make for a flatter finished piece with many fewer bubbles.

Wednesday, 5 April 2017

Large Bowed Pieces

Occasionally, large pieces in the kiln develop a bow at the end of firing.  The most obvious is when the bow is upwards, but it also occurs that the piece is domed.  This is much more likely to be observed when there are complete sheets, rather than ones interrupted with other design elements which break up the whole sheet.

This is a result of a slight mismatch of compatibility.  One glass is expanding and contracting slightly more than the other.  The bow is always toward the glass which expands the most.  When it contracts, it also contracts more than the other glass, drawing the sheet with lower expansion toward it to form a bow.

This is a form of mild stress.  It can sometimes be seen in large sheets of streaky or flashed glass which are not completely flat.

It is not a fatal flaw.  A piece of this nature can survive many years in that state.  I once had a large window to repaint, because of a football impact.  When re-assembled, it showed that it had been bowed from the outset, almost 90 years before.  It is not in a suitable state for wall pieces or other things that need to be flat, of course.


The remedies most often relate to reducing the stress in the piece.

This of course, relates to the firing schedule.  Increasing the length of the soak at the annealing point is one method.  This combined with reducing the rate of cooling can be effective.

Another method can be employed also.  This is to soak the glass just above the upper strain point of the glass.  This soak should be equal to the one planned for the anneal.  The upper strain point temperature – that point above which no annealing can occur -  is about 40C above the annealing point.  Thus, this soak should occur about 55C above the annealing point of the glass concerned.  Then proceed at a moderate pace to the annealing point.  This rate may be the same as the second stage of the anneal cool (as a starting point). Then anneal as usual for the thickness of the piece.  This method can, of course, be combined with the extended soak and reduced cooling rate as first suggested.

A third method can be employed, if the first two do not work.  This assumes one of the sheets of glass is clear.  Place a sheet of clear on the opposite side of the piece to form a glass sandwich with the two pieces of clear.  Then fire as for a three-layer piece of glass.  The assumption behind this is the same as for toughened glass.  The outer layers will hold the inner layer in compression.  But more importantly, will equalise the slight stress, allowing the piece to remain flat when the firing is completed. This can be used with any transparent glass, but the colour change may not be acceptable.

A fourth method is possible.  Turn the fired piece over and fire, to allow the weight of the glass to overcome the tension of the contraction of the more expansive glass.  This can be successful, but it does retain the stress within the resulting piece.  As such it is not a remedy for the stress, but is a way of flattening.


The place of the glass in the kiln can have an effect too.  If the sheet is near the side of the kiln, there can be a stress inducing effect.  All kilns are a bit cooler at the perimeter than at the interior.  This applies to circular, oval and rectangular kilns.  Rectangular kilns have additional cool spots at the corners.  If the glass is near the capacity of the kiln, the cooler corners can induce this bowing stress to otherwise compatible glass.  The thing to do is to stay about 50mm away from the edges of the kiln when firing large sheets into one piece.


The ideal is to know before firing the large piece whether there will be a problem to overcome. This requires a simple test of the glass to be used.

Assuming the final piece is to be two layers thick of different glass colours, cut a strip of each colour about 50mm wide and as long as the final piece.  Assemble them in the same order as you plan for the final piece.

Add an annealing test square of the two glasses stacked on top of one another.  If one is opalescent and the other is transparent. Make the transparent larger than the other.  If both are opalescent, you will need to run a compatibility test at the same time as this test.  In simple terms, it is to put each of the opalescents on a strip of clear or transparent with the gaps between the opals filled with the transparent.  This test will tell you whether you have fired so fast as to induce stress and so invalidate the test.

Fire as though for a 50mm piece of jewellery – about 200C to bubble squeeze - but without a soak - and then at 400C to top temperature.  Cool to annealing temperature for 15 minutes and cool at 120C per hour to 370C and turn off.

If the long strip is bowed, and the anneal test piece shows no stress, there is enough compatibility mismatch to require the use of one of the remedy methods outlined above for the main piece. It may of course, cause a reconsideration of the glasses to be used or the size of the piece.

Wednesday, 29 March 2017

Colourising Metals for Glass

There are many minerals and chemicals that are used to give glass its variety of colour.  This note attempts to give information on the most common elements and combinations used to impart the colours to the glass.

Antimony oxides produce white glass as do tin oxides.  Together with lead, antimony results in yellow.

Cadmium together with sulphur forms cadmium sulphide and results in deep yellow colour.  Together with selenium and sulphur it yields shades of bright red and orange. 


Chromium is a very powerful colourizing agent, yielding dark green or in higher concentrations even black colour. Together with tin oxide and arsenic it gives emerald green glass. Chromium aventurine, in which aventurescence was achieved by growth of large parallel chromium(III) oxide plates during cooling, was also made from glass with added chromium oxide in amount above its solubility limit in glass.

The material can be introduced into glass either in the form of chromic oxide or potassium dichromate, the latter being a more convenient form.  Potassium chromate is yellow and this colour can be imparted to certain glasses. To produce emerald green glass in which a yellowish cast must be avoided, the addition of tin oxide and arsenic is necessary.

Chromium is associated mainly with the production of green glass, but other colours from yellow through bluish-red, red to dark green or even black can be achieved in combination with other oxides.

Cobalt is the most powerful blue colorant used in glassmaking producing rich blues when used in potash containing mixes, but it can also give shades of green when used with iodides.  The deepest of blues are produced when used in glass containing potash.

Small concentrations of cobalt (0.025 to 0.1%) yield blue glass. The best results are achieved when using glass containing potash. Very small amounts can be used for decolourizing. Addition of 2% to 3% of copper oxide produces a turquoise colour.

Copper is a very powerful and versatile colouring agent when used in colouring glass.  Copper greens and blues are not difficult to produce, although the behaviour of copper in a silicate melt can be complicated.  Copper was used most profusely to produce green glass.  

Pure metallic copper produces a very dark red, opaque glass, which is sometimes used as a substitute for gold in the production of ruby-coloured glass. The art of using copper for ruby glass goes far back to ancient times but even so using copper oxide to make ruby glass can be very difficult. Today we find copper being used to produce turquoise blue tones.

Didymium gives green colour (used in UV filters) or lilac red.

Metallic gold, in very small concentrations (around 0.001%, or 10 ppm), produces a rich ruby coloured glass (gold ruby), while lower concentrations produces a less intense red, often marketed as cranberry. The gold is used as gold chloride. The colour is caused by the size and dispersion of gold particles.

Iron is a very useful and powerful colouring agent even though it can be an undesirable impurity in making glass. Iron in its metallic forms cannot remain in equilibrium with glass and can be disregarded. But its ferrous and ferric forms are of a great help in producing coloured glass.  Iron(II) oxide may be added to glass resulting in bluish-green glass.  

Together with chromium it gives a richer green colour.  In a reduced condition, it can be combined with chromium to produce a deep green glass. 

Used with the combination of carbon or other reducing agents, sulphur and iron sulphides give a dark amber colour.

Lead compounds produce a range of yellows.

Manganese can be added in small amounts to remove the green tint given by iron, or in higher concentrations to give glass an amethyst colour.

Manganese dioxide, which is black, is used to remove the green colour from the glass. This results in a very slow chemical process where it is converted to sodium permanganate, a dark purple compound. Windows made with manganese dioxide solarise to change to a colour which is lightly tinted violet because of this chemical change.

Manganese in its low state of oxidation is colourless, but it is a powerful oxidising agent and can be used for decolourising purposes to oxidise the iron content.  Manganese is mainly used in the production of purple glass resembling the colour of potassium permanganate crystals. The purple colour is achieved by the trivalent manganese however in its divalent state it only imparts a weak yellow or brown colour.

Nickel, depending on the concentration, produces blue, or violet, or even black glass. It is used in the production of smoky coloured glass

Lead crystal with added nickel acquires purplish colour. Nickel together with a small amount of cobalt can be used for decolourizing of lead glass.  When it is introduced into lead crystal it gives a purplish colour, which compensates for a yellow tint produced by other constituents.

Selenium, like manganese, can be used in small concentrations to decolourize glass, or in higher concentrations to impart a reddish colour, caused by selenium nanoparticles dispersed in glass. It is a very important agent to make pink and red glass. When used together with cadmium sulphide, it yields a brilliant red.

Silver compounds such as silver nitrate and silver halides can produce a range of colours from orange-red to yellow. The way the glass is heated and cooled can significantly affect the colours produced by these compounds.

Sulphur, together with carbon and iron salts, is used to form iron polysulphides and produce amber glass ranging from yellowish to almost black.  With calcium it yields a deep yellow colour.  

In borosilicate glasses rich in boron, sulphur imparts a blue colour. Cadmium sulphides, which have a deep yellow colour, are often used in the production of glazes and enamels.

Tin oxide with antimony and arsenic oxides produce an opaque white glass (milk glass), first used in Venice to produce an imitation porcelain.

Adding titanium produces yellowish-brown glass. Titanium, rarely used on its own, is more often employed to intensify and brighten other colourizing additives.

Uranium (0.1% to 2%) can be added to give glass a fluorescent yellow or green colour.  Uranium glass is typically not radioactive enough to be dangerous.  It is often referred to as Vaseline glass by USA collectors.  When used with lead glass with very high proportion of lead, produces a deep red colour.

A good visual chart of minerals and resulting colours is here.

Influence of the glass-making process on colour.
It is not only the minerals that give the glass the colour; it is combined with the way in which the materials are treated.  The physical conditions under which the glass is made also have an influence on the colour.  The main ones are:
1.   The temperature of the melt/batch
2.   Temperature of reheat during the working of the glass
3.   The temperature of the 'Lehr' (Annealing Oven)
4.   Duration of the melt/batch
5.   Time and temperature relationship at different stages in production
6.   The type of colorant being used
7.   Concentration of the colorant
8.   Atmosphere of the furnace
9.   The composition of the colorant within the glass composition, as is the case when iron is added to glass. The type of iron oxide formed decides if the glass will be blue or yellow
10.                The number of times the same glass is melted. Repeated melting of the cullet will usually give a darker tone to the finished piece