Glass Stains

A lot of people want to use old window glass – for recycling reasons, for the character of the older glass or because it is cheap or free. This glass often has stains or the appearance of corrosion or etching from age or storage conditions. To remove these blemishes you should start from the least aggressive method and work your way through the more aggressive methods until you find one that achieves the cleanliness you desire.


When using these methods appropriate breathing and hand protection are a requirement. 

Surface deposits

The first method is to wash the glass in water with a bit of crème cleaner. Place the glass flat on a work bench and scrub it with a brush containing the water and cleaner. Rinse and dry. This may be all that is needed.

More persistent stains require chemicals. Use rubber gloves for this kind of process. Soak the glass in a dilute (5-10%) solution of lye. This is also known as caustic soda. Chemically it is sodium hydroxide (NaOH). After the soak wipe with a disposable towel and dip it into a dilute solution of hydrochloric acid (5-10% again) or vinegar to neutralise the caustic soda. Wipe the glass with a towel and rinse with plain water and polish dry.

Metallic stains or iridescence and etched surfaces

If the blemishes are more than surface deposits, more aggressive methods are required. The metallic-like stains and iridescence are usually evidence of the corrosion of the surface of the glass. These and any etching require grinding and polishing.

You should start with a polishing process to determine if that will be sufficient to bring clarity back to the glass. The use of an optical polishing material such as cerium oxide paste and a felt polishing head is often enough. Ensure that you do not let the cerium oxide get any drier than a paste to avoid localised heating and therefore breakage of the glass. Rinse the glass in clean water and polish dry.

If this does not remove enough of the surface to provide the desired clarity of the glass you can try either using pumice or jeweller's rouge, or dilute hydrofluoric acid.

Hydrofluoric acid used in a 10% or less solution will remove the surface of the glass, so eliminating the evidence of corrosion. It will even out, but not eliminate, the evidence of any etching. The disadvantage to this method is the risks associated with such a dangerous chemical. There is within this note on acid etching a door panel some advice on safety.

If you decide to avoid the hydrofluoric acid method, you can use mild abrasives such as jeweller's rouge or pumice with water and a felt polishing head. Once you have ground the whole of the surface, you need to wash it very thoroughly in clean water. Then change the felt head and go to cerium oxide to provide the optical finish.

Note:

All the dilutions and solutions suggested here are with water.

This note assumes the glass has had any paint already removed. For a method of removing house paint see this tip.

Negative assembly

To get a crisp design finish to a fused piece especially with thin elements like stringers it is often recommended to fire upside-down. This means that the bubbles move toward the final bottom of the piece rather than rising and disturbing the design on the top.

Assemble the piece on your workbench as usual on top, but use a toothpick or small brush to dab some GlasTac or other glue near the two ends of each piece. Let it dry overnight and then carefully flip the whole arrangement upside down onto the kiln shelf.

If you don't like the glue and flip over technique you can try another. Draw your design onto Thinfire shelf paper in negative with a graphite pencil. If you have a strong light source you can draw on the reverse (printed) side and trace the negative onto the upper face of the Thinfire. Then assemble your pieces upside-down on the shelf. You can assemble the whole in the kiln and there is no need for glue.

When fired, clean the piece thoroughly and decide whether you want that texture or a smoother surface. If you want the smoother surface, put in the kiln for a fire polish. This is often known as “flip and fire.”

Soldering Zinc

Brass, copper and zinc are heat sinks. That is, the metal conducts the heat rapidly so more heat has to be applied than for lead and tin to keep the soldering site hot enough to accept the solder.

The important elements are:

Use a hot iron. If you use a rheostat, turn it up to full. If you can, change the tip/bit to one rated at 800F – it will have an “8” stamped on the end that goes into the barrel of the iron.

Apply the flux liberally at the soldering point to ensure the area is “wetted”.

Keep the iron in contact with the came for a few seconds to heat the metal. If you are using zinc as your border around a leaded panel, make sure you do not heat up the lead came so that it begins to melt.  Keep the iron on the zinc.  It will transmit heat to the lead came without getting it hot enough to melt.

When the came is hot, apply the solder to the bit. 

Keep the bit on the metal until you see the solder begin to flow on both the zinc and the lead or onto the solder bead (on copper foil), then gently lift directly up. 

Starting

“Where should I start cutting the glass from the inside or outside of the cartoon?”

Once the colour selections are made, you can start the cutting in what ever order suits you. Some cut colour by colour. Others cut the glass in the order that fits together. This usually means the main image is cut first and the pieces laid up on a glass easel to determine their interactions. When satisfied, the background or in-fill pieces are cut and fitted.

“Where should I start building a panel?”

When starting the assembly, begin at a lower corner. Whether left or right will depend on whether you are right or left handed and the way the panel fits together. Although sometimes it can be easiest to build from the top toward the bottom. Starting in a corner, together with battens or supports for the outer edge  will keep everything in place as you work.

When working on an oval or circle, you can use a multiplicity of nails around the outside edge to hold the glass pieces. An alternative is to cut the curve into a piece or pieces of float glass and fix this to the work board to hold the glass. If you do this use the outside of the cutline so that there is space for the heart of the perimeter lead.

Separators for Steel

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

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

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

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

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

Single Layer Slumping

Contrary to expectations, single layers are more difficult to slump than multiple layers, as you do not have so much mass for the gravity to act upon. Especially if the pieces are small, the slump will be slower than for large pieces.

It is possible to take the glass up in temperature very quickly without breaking it, but that will not give you much control over the slump. You may not get the curve you want, and you may over fire it so that it distorts.

If you feel the need to go quickly, slow down from 630C to a rate of advance of 40 or 50C/hr. For the first firings and new moulds, observe how the glass settles into the mould. This will give you the top temperature.  In future you can back off 10C or more and soak for about 30 minutes to get less marked pieces.

You can drape or slump a single layer over a mold or drape two overlaid layers on or over a mold – assuming the layers are 2 mm to 3 mm thick.  Draping double layers together is a popular way to make two colour double layered handkerchief vases with art glass. But when you fire one piece of glass on top of another, the air between the two pieces acts as an insulator, slowing the heat transfer between the two pieces of glass. So you should advance at about half the rate you use for a single layer. Anneal soak and cool for 6 mm glass.

Note that there are different behaviours between single and double layered pieces.

Draping and slumping processes work by taking advantage of the action of gravity on the just pliable glass. This means that thicker, heavier glass slumps and drapes more quickly than thin light glass does. You will need longer soak times for thin glass than for thick glass. You can go higher, but the effects become less controllable.

The size of the opening of the mould has a strong effect when slumping single 3 mm layer glass. You need much more time – often as much as three times as long as for a 6 mm piece over a small diameter.

The effect of the shape of the mould has little effect on simple shapes. But when angular or complex curves are part of the mould the single layer will have much more difficulty in conforming to the mould than the thicker pieces.

Annealing the 3mm piece can be a bit quicker than for 6 mm, but a reduction of more than a third in the soak time and more than doubling the cooling rate may cause problems.

Short Cuts in Fusing

There really aren't any. Trying to do two things such as fusing and slumping in a single firing requires compromises that can make the result unsatisfactory. Normally, trying this uses more time and glass in having to re-do things.

Referring to what heat does to glass and when is important in thinking about what you are trying to achieve. Bullseye in their tech notes give the information in both concise and visual ways.

Fixing a Bubble

It is often recommended that large bubbles can be fixed by drilling out the hole if the raised part is thick, or by breaking the thin skin of the bubble and filling with a piece of glass or frit.

My experience leads me to say that "drilling and filling" is hardly ever successful. There always is a mark where the fill has been made, often there is a colour variation too.

There is another possible solution for small bubbles which are at the surface but not yet burst.

Turn the whole piece over and take to just below full fuse. This will allow the bubble to begin moving to the "bottom" of the final piece. Then flip over, clean really well and  fire polish - quickly up from bubble squeeze temperature and with no soak - to give a smooth surface, but not allowing it to get so hot the bubble comes back to the top. This will require observation to stop the temperature rise as soon as the surface is shiny.

Small burst bubbles

Small burst bubbles can be fixed by placing small “beads” of glass made from frit in the kiln. The appropriate sized bead can be placed in the hole and others arranged in a pleasant arrangement and all taken to a tack fuse. The beads are made by cutting small squares of glass 5-6 mm and taking them to full fuse. The glass will round up at the fusing temperature and give a number of completely rounded spheres with a flat bottom.

Prevention

Of course, the prevention of bubbles by inserting a bubble squeeze is the best solution.

revised 24.12.24

Resists

Avoiding bubbles between the resist and the glass on large sheets when preparing etching or other resist based processes is often difficult.

A trick learned from the firms that apply advertising vinyl to vehicles is to use a spray filled with slightly soapy water.

Their process uses pre-cut vinyl with an adhesive backing. So the first thing to do is to pick out the unwanted pieces. That is the pieces covering the areas that will be etched. Then you need to put a backing onto the front surface of the picked out vinyl – usually really wide masking tape.

Lay out the vinyl on the glass. Tape one end of the vinyl securely to the glass. This ensures that you get the vinyl correctly aligned over the whole area. Fold the whole piece of combined vinyl and backing back to the taped edge.

Carefully peal back the covering for the adhesive side making sure you do not pull off any of the isolated vinyl pieces. Spray the glass with a mist of soapy water to ensure all the glass is covered, do not have the glass running with liquid, but be generous. Too much liquid means that there will have to be a long wait for the vinyl to be firmly attached to the glass.

Start the application process by folding the vinyl onto the glass. Use a credit card or better, a large squeegee such as used for grouting mosaics. The tool you use must be smooth to avoid scratching the vinyl. Push the soapy water forward and to the sides as you move along the piece of glass. Keep pulling the protective layer evenly off the adhesive side as you work forward.

When completely attached, remove the backing from the vinyl. This will enable you to see any bubbles you may have left. Work out any bubbles by further pressing the soapy water out from under the vinyl to the edges. Where any remaining bubbles are in the way of the design, puncture them and work out the bubble of moisture through the hole. Cover the puncture with a small piece of vinyl.

Leave for a day for the vinyl to become firmly attached to the glass and then you are ready to do the etching.

Where you are going to cut the vinyl by hand, you do not need the backing. All the rest of the process is the same.

How Annealing Affects Slumping

It is often claimed that inadequate annealing of the fused blank can cause breaks during a slump firing.

If annealing is the cause, it is likely to break on the rise in temperature.  Once the piece has reached the annealing temperature, any breaks will be due to thermal shock on the way down.  An annealing break usually has a hook at both ends of the break, although this is more difficult to determine in a shaped piece.

Thermal shock tends to be along straight(ish) lines, often between thick and thin, or strongly contracting colours.  It tends to happen on the cool down. 

Breaks on the rise or fall in temperature are difficult to distinguish on slumps.  The temperature is low enough that there is little to distinguish the sharpness of the edges.  The real method of determining, is to try to fit the pieces together.  If they fit exactly, the break was during the cooling.  If they have even a little variation in fit, the break occurred on the rise in temperature.

If the annealing of the slump is marginally inadequate, it may break hours, days, weeks after cool.  The less stress the longer it will survive.  This will not be the result of any inadequate annealing of the fused blank. Only the last annealing is relevant to the soundness of the piece.

How can you ensure the annealing on a slumped piece is adequate?

You need to check the fused blank for stress before slumping to ensure it has no or very little stress.  The anneal for unstressed items needs to be at least equivalent to, or longer, as for the fused blank.  Testing has shown that annealing for one layer greater than the calculated thickness results is less stress than annealing for the thickest part.

Fire more slowly than usual for blanks with moderate stress to avoid thermal shock during the first ramp.  Anneal the slumped piece for one layer more than was done for the blank.  There is no other requirement for slower annealing.

Pieces with significant stress need to be returned to the kiln to be annealed.  Fire them up significantly more slowly than you normally would for a piece that thick.  This may be one half or less the speed used on the un-fused pieces. Anneal as for a thicker piece.