Bottle Slumping

Placing

Production made bottles are not evenly thick all the way around. So they roll until the heaviest part is at the bottom.

One way to avoid any rolling in the kiln is to put the bottle on a level surface and let it roll to the heavy side. Mark the up side so that you can place it in the kiln with the heavy side down.

Some times though the seam lines – which will show in the final piece – are in the wrong place. To keep the seam lines to the side, you can place a small piece of 0.5 mm fibre paper against each side of the bottle at the base if you are working on a hard surface.  On a softer surface, the weight of the bottle or a gentle push will sink it into the surface enough to avoid rolling.

The distance to place bottles apart is important to know so they do not become attached when several are slumped together.

You can slump test bottles of various diameters to determine their final width.

However, if you wish to calculate the distances, The width of slumped bottles is approximately 1.6 times the diameter of the original bottle. If you want the base of the bottle to be flat too, the final size will be wider, dependent on the amount of glass in the base.

You also need to leave some additional space than this calculated distance between bottles, as the final size is narrower than the size when hot. You should leave at least an additional 10 mm to each side of the bottle.

So a 50 mm diameter bottle will become 80mm wide. You need an additional 10 mm each side, so the spacing from the centre of one bottle to the next should be at least 100 mm.

The surface that you are slumping onto is important too.

You can place the bottles on a sand base that has been dusted with kiln wash powder. This has the advantage of allowing a gentle push into the sand to prevent rolling. But it imparts a grainy texture to the back.

You can get a smoother surface by using whiting or sifted plaster that has been screeded smooth. This also allows the bottle to be gently pressed into the surface.

You can use fibre papers as separators from the shelf, but they are relatively expensive.

Kiln wash works very well. It can provide a very smooth surface, but if you want more texture, you can sprinkle some kiln wash through a sieve over the shelf.

Bottle Slumping

There are four major considerations in bottle slumping: cleaning, placing, firing, results.

Cleaning

A major consideration in slumping bottles is the cleaning required. This requires a lot of time, as everything has to be clean inside and out.

Labels and the glue attaching them must be cleaned off. The interior needs to be clean. And the bottle needs to be dry before being placed in the kiln.

Soaking first helps the cleaning process. If you are doing large numbers you will need to find large containers that you can leave the bottles in to soak for a couple of days. For small numbers, a few bottles soaking in a bucket of soapy water will do. The bottles should be upright to allow the internal residues to float to the top. After a few days the labels should have fallen off and the internal deposits floated to the top or easily washed out. Screen or plug the sink to be able to remove the residues from the sink before it gets into the drain. Then you can proceed to rinse and clean the bottles.

The bottles will need to be wiped free of the glues used to stick the labels to the bottles.  Any glue residue left on the bottle will show up on the finished piece, usually as devitrification. If the glue or label has not come off with the cold water and soap soak, you can soak them in very hot water. You can then use a variety of solvents to remove the most persistent adhesives.

For the difficult internal deposits, you can use one or two short lengths of small chain and slosh that around with water. The chain used for hanging small stained glass panels is ideal.

As you can see the cleaning process is lengthy and can be time consuming. So you might want to see if you can get new bottles at a reasonable price. Home brew shops may have bottles they are willing to dispose of. Bottling plants may also have supplies of bottles. New bottles will greatly reduce the labour of bottle slumping, although it does not fit the re-cycling ethos that brings people to bottle slumping in the first place.

Hinges

What are hinges in stained glass?

As the glass is the strongest part of a leaded or copper foiled panel, the joining materials (lead, solder) are the places where the panel can flex. If you have long straight or nearly straight lines extending to or toward the edges of the panel (vertical, diagonal  or horizontal), this is the place where the panel can completely bend (hence "hinge"). It provides a danger in handling that the panel will break. Even if the lines do not run all the way to the edge, any significantly long line will put pressure on the glass pieces at the ends of the hinges, such as a series of formal border pieces or narrow central pieces. It will be a weakness in the long term whether it survives the studio processes or not.

However, we all have seen leaded glass windows with single or multiple hinges that survive for many decades, and only as they loose support from firm cement and the ties to the saddle bars break away, does the panel begin to self destruct.

It is important to recognise where these hinges are to be able to place reinforcements on the panel.

But the real solution for making a panel that will last, it is best practice to avoid designing hinges into the finished work.

Silicone Removal

Removing silicone residues from glass is difficult as silicone combines with the surface molecules of the glass.

The best advice seems to use a very sharp flat blade similar to a paint scraper. There is a tool which holds razor blades or Stanley knife blades which is suitable. Keep the blade as near parallel to the glass surface as possible. Take thin slices of the silicone away at each pass. Keep the blade free from nicks. Change the blade frequently. You may be able to remove almost all the silicone, depending on the smoothness of the glass surface, in this way.

However when you have removed as much as possible by the mechanical means, you may need to use a chemical process to get rid of the remaining silicone. Silicone dissolvers are available from most do-it-yourself shops. Make sure you use them in accordance with the instructions as they can be mildly toxic.

Space In Copper Foiled Projects

When cutting for copper foil projects it is important to leave a small gap between the pieces. This is both to allow for the thickness of the foil and for the solder to form a bridge to the other side of the panel.

I prepare the space in the cutting process by scoring at the edge of the pencil thin cartoon line. This leaves the thickness of the line between the pieces. I do not use pattern pieces unless I am using very dense opalescent glass. When I do use pattern pieces I cut them out with a scalpel knife so there is no space between the pattern pieces. I then draw around the pattern pieces and cut to the inside edge of the drawn lines. This also gives a margin for the foil.

Fitting the pieces is still required though. Some people foil one piece and then fit the next to it before foiling it. This is probably the most accurate way of getting a close fit. However, I save up my cut pieces which I have fitted to the cartoon and foil them while watching TV. My cutting and fitting is normally accurate enough that I do not have to take the foil off pieces, grind and re-foil.

While doing this fitting you need to be aware that an additional small gap between pieces is required to allow the solder to join both the front and back of the panel. This forms an “I” beam for strength. The adhesive of the foil is not enough to make a lasting and strong panel. The solder joining the two sides will give the panel the strength to last for a long time.

Stuck Beads

Sometimes beads refuse to come off the mandrel with normal twisting of the bead on the mandrel. In these cases I use two methods.

I lock a small vice grip (locking pliers) on the mandrel near the middle and hold bead with non slip material. Then I twist back and forth to loosen the bead on the mandrel. When it frees from the mandrel, I pull toward the end continuing to twist.

If it does not loosen, or come off the end, I soak the whole in water over night and then repeat the above procedure again.

If these two do not work, I sacrifice the bead, as even using soft jawed vices normally leads to the crushing of the bead.

Thick Uneven Pieces

Occasionally fused pieces come out of the kiln with one side thicker than the other. There are several things that need to be done for the present piece and for the future.


Level

First check how level your kiln is. The best for this is to begin with a check of the bed of the kiln. Check the level in four directions – left-right, front-back and the diagonals. If it is practical, wedge up the legs of the kiln to make the bed of the kiln as level as practical.

Then check how level your shelf is. Put in your shelf supports and then place the shelf on them. Again check with a spirit level the four directions. Place pieces of fibre paper under or on top of the supports to level the shelf. It is only after these checks have been made that you can consider firing your piece to help it return to an even thickness.  As part of your kiln maintenance you should check the level of your shelf at least monthly, if not every time you prepare to fire.

Variation in Thickness
Now that you know the shelf is level, you need to consider what the variation in thickness across the piece may be. The firing schedule needs to be more conservative than just for the thickest part. As the thinner parts will heat through more quickly than the thickest parts, you need to fire less quickly than you normally would for the thickest area. A rule of thumb – not always correct of course – is to add the difference of the thick and thin areas to the thicker and fire for that calculated thicknesses. This will make the firing schedule slower and so allow the thicker part to be the same temperature as the thinner. For example, a piece 6 mm at one side and 10 mm the other would have a difference of 4 mm. Add this 4 mm to the thicker 10 mm and then fire for 14 mm.

Temperature and Soak
You also need to consider the top temperature to use and the length of soak required. Glass flows relatively slowly at kiln forming temperatures. The conservative approach – one that allows further work if necessary – is to use the previous fusing temperature and extend the soak by at least twice the length of time on the previous firing, even perhaps to a couple of hours.

Bubbles
One thing that will happen is that the bubbles that previously were near the surface will rise and burst giving pin holes on this extended soak. So you should consider cleaning the bottom and putting the top face down on a separator between the shelf and the glass.   This will reverse the direction of flow for the bubbles. Few if any will break through the new top and there should be no pin holes when flipped.

Further Firings
When the piece is cool, check it for the even-ness of the piece all around. If it is not even enough, you will need to consider re-firing again. If you decide to do so, you should go no faster than the rate of advance as previously – probably even slower - but consider raising the temperature or extending the soak. Remember that achieving the heat work required at the lowest temperature is the guide line for kiln forming. So an extended soak should be preferred over a higher temperature, unless there are strong indications that a higher temperature is required.

Fire Polishing
Of course, you will now need to throughly clean the face down side and re-fire to fire polish the original top. The rate of advance should be the same or slower than the firing to even the thickness. Once you have achieved about 600C, a soak of about 30 minutes will ensure that the glass is thoroughly heated through. Then you can advance at a quick rate to the fire polish temperature with a soak of no more than a minute. This allows the surface to change without giving the rest of the glass time to begin to move.  Of course, a thorough annealing is required.

This procedure for re-firing  can be used when re-firing pieces for any reason. You only need eliminate the considerations on the uneven thicknesses.

Wire for Hanging

The most common wires used for inclusion in fused objects are copper, brass, nickel/chrome, stainless steel and sterling silver.

The strength of the wires – strongest to weakest - seem to be in the order of stainless steel, nickel/chrome alloy, brass, silver, copper. The metal you choose will be related to the weight of the piece, the available thickness of wire, and aesthetics.

All of these are subject to fire scale or fire stain, a blackened surface on the wire. This can be removed by abrasive cleaning of the exposed metal. The metal within the glass most often takes up the fire scale too. This can be reduced by thorough cleaning of the metal before enclosing it in the glass. Coating the metal with a flux such as borax often reduces the incidence of the fire scale too.

The techniques of cleaning the fire scale from the metal range from scrubbing and polishing to tumbling. The tumbling has the advantage of hardening the softer metals such as copper, and silver.

Copper looses much of its strength in the firing, and often needs gentle working to stiffen it. This is where tumbling is so useful.

Pure silver normally leaves a yellow stain on the glass. Sterling silver - an alloy of copper and silver – is less inclined to do this. However the exposed wire will stain the shelf and any subsequent glass unless well supported by 1 mm or more of fibre paper.

It is common in silversmithing to pickle silver to remove the fire scale after any heat work.

Complex Drapes

A question has been asked that relates to draping over a face mask mould, but this response relates to any drape with compound or multiple shapes.

The rate of advance should be a steady one all the way to the forming temperature. This should be 150C/hr or less. Thicker glass requires a slower rate of advance to allow the glass to heat all the way through. The point is to get the glass all the same temperature by the time it reaches the forming temperature.

Draping over an undulating mould takes more heat or time or both than simple drapes, because there is a much greater variety of form for the glass to conform to.

You have a choice about the top temperature and soak times. You can choose a low temperature with a long soak time, which most often leads to a minimum of marks on the underside of the piece, but requires long vigils at the kiln to determine when the drape is finished. The other strategy is to go for a higher temperature and shorter soak, which leads to more marks, but less time observing the firing. The higher temperature may be as much as 720C.

Note that there are a group of considerations about the size of the drape and the thickness of the glass being draped. 

Weight 

Span 

Complexity of shape 

There are some things you can do to assist the progress of the drape. One, already mentioned, is to increase the forming temperature.

Use a longer time, or as much time as required. Watch the draping progress because it is never certain how long the piece will take to conform to the mould sufficiently.

Use props. Place the props where the glass first starts to form just barely supporting the edge of the glass. As the glass begins to bend, it will slip off the supports - assuming they are well covered in separators, even pieces of kiln paper. This means the folds will start somewhere else than at the nose on a face mould or other high point on any other form.

Manipulate the glass. Reach in with wet wood sticks and push the glass about. One stick will be needed to keep the glass in place while the other pushes the glass about. The sticks do need to be both wet and strong. If you use dry sticks they will mark the glass as well as go on fire. Of course, you need protective gear to avoid burns to your skin and hair if you do this.

Mitred Corners

There are various ways to determine where and at what angle to cut the lead came, especially for panels with more than four corners. But the most common is a four cornered panel. The following method works for that circumstance very well.

My preference is to use the battens surrounding the cartoon as a guide for the placing of the mitres on the first two cames. The battens are placed around the cartoon to suit the came width. The vertical came is placed in contact with the bottom came.

A short piece of came – ca. 50 mm – is placed on the external cartoon line almost in contact with the vertical lead. A second short piece is placed on top of the short piece and extended over the top of the side lead.

Mark the lead lightly with a nail or pencil. Cut the angle from the inside to the outside corner. This will form a 45 degree angle.

You can do the same for the bottom came, by removing the vertical came for the time being and doing the same operation on the horizontal came. Then they can be place back together for the next operation.

The finished mitre

To determine the length of the came which is already mitred on one end, put the came in place on the cartoon.

Then place a short piece of lead on the cut line which is at right angles to the came to be measured. Then place a short piece over the two cames as for the first mitre cut.

A nail or other pointed implement is used to scratch a line on each side of the overlapping lead. When this overlapping lead is removed, a diagonal is drawn from the inside mark to the outside mark. Cut along this diagonal.

Ready for cutting

There are other methods for panels with more than four right angles.