Flashed Glass

Red pot metal glass is often undesirably dark in colour and very expensive. The method developed to produce red glass was called flashing. In this procedure, a semi-molten gather of coloured glass was dipped into a pot of clear glass. As the bubble became enlarged, the red glass formed a thin coating on the inside. The formed glass was cut, flattened and annealed as any other blown sheet.

There were a number of advantages to this technique. It allowed a variety in the depth of red – and other deep colours - ranging from very dark and almost opaque, and sometimes merely tinted. The other advantage was that the colour of double-layered glass could be engraved, abraded, or etched to show colourless glass underneath. 

Other base colours are also used in making flashed glass, for example red flashed onto a pale green base.  Also see this post on finding the flashed side of glass.

There still exist a number of glass factories, notably in Germany, USA, England, France, Poland and Russia which continue to produce high quality glass by traditional methods primarily for the restoration of ancient windows.

Slumping Etched Glass

In general slumping will not remove the evidence of etching. There will be very little effect on etching on the bottom even at fire polishing temperatures.

When the etching is on the top side exposed to the radiant heat of the elements, you need to be careful to use the lowest practical temperature for slumping. It is possible to achieve a satin finish to a sandblasted surface at 677ºC with a soak of two hours. It depends on the delicacy of the etching texture as to whether the slumping will affect it much.

The more the glass will need to move during slumping the more distortion will be apparent in the finished piece. This can be minimised by using a low heat for considerable time.

If the mould is very detailed, it would indicate that etching should be done before the slumping due to difficulty in attaching the resist to the shaped glass, unless you paint it on. But again, a significantly long soak will be required to achieve the detail of the mould.

If it is a simple and relatively shallow slump it may be easy to etch after shaping. It is a question of how easy it is to get the resist to conform to the curve.

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.

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.

Resists for Large Areas

Avoiding bubbles appearing under the vinyl resist on large sheets of glass when preparing etching or other resist based processes is often difficult.

A trick I learned from the firms that apply advertising vinyl to vehicles is to use a spray bottle filled with water that is just soapy. A few drops per pint will be sufficient.

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 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.

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.