Fusing

What is fusing?

Fusing is the “melting together of two or more layers or pieces of glass which may be of similar or of widely differing size or shape, and may include flat glass, rods or threads, crushed or broken glass”.

Whatever the combination, for a successful result all the pieces should have the same coefficient of expansion (COE);  they should be compatible.

Canberra Glassworks, fusing class

Tack or full fuse

Fusing glass relies on the force of gravity to press the pieces together, they are best laid in a flat or near horizontal position.

There are two stages in  fusing:

• Tack or stick fuse
• Full fuse 

When glass is being heated toward a full fuse the contacting faces will soften and stick together in patches, hence the term tack fuse.

The join remains visible but the pieces cannot be separated. It occurs progressively as the temperature rises: first at one spot, then some patches and finally all over.

The extent to which a tack fuse is visible will vary with the clarity of the glass and the point where the firing was halted.

Further increase of temperature causes the skin on the mating surfaces to dissolve, creating an homogeneous whole. There will be little indication of the original make-up of the piece, other than at the edges.

Tack fuse is an intermediate stage in all full fuses but a firing is sometimes halted at this point: maybe because an artist wants the particular effect, or wishes to tack some pieces together prior to further processing. 

At fusing temperatures the glass is like a thick fluid. It will flow off high spots onto lower spots, find its own level if placed as a sheet on an out-of-level shelf, flow into crevices: do the most surprising things under the force of gravity.

The bottom surface of the glass will be soft enough to take up texture from whatever supports it. Thus, when doing a full fuse a flat slump is also taking place.

Glass arrangements for full fuse

There are numerous ways in which pieces of glass can be arranged for fusing. They can be:

• layers of similar size or shape;
• layers  of dissimilar size;
• a segmented layer combined with a single piece layer;
• single segmented layer fused at mating edges;
• small pieces sandwiched between larger pieces.

Fusing like size pieces

In this arrangement pieces of similar size or shape are laid one on top of another. There can be any reasonable number of layers but commonly there will be two. The layers can be of  float, of compatible fusing glass, or of any type of glass one may try. They can be quite small, or as large as the kiln will accept.

The fuse may be to make thicker glass from thin sheets, or to combine layers of different colours, and can involve glass of the same or of different thickness.

Materials such as nickel, silver or copper wire, silver or gold foil, leaves or other objects can be fused between two layers of clear glass.

The most significant consideration is the effect of heat and oxidisation on the introduced material and the effect of differential contractions during cooling.

When fusing two pieces of similar size, the objective is for there to be minimal evidence that a fuse has taken place. Some of the indicators of the success of this include optimum clarity; no bubbles, top surface smooth, and the bottom surface showing texture of shelf or underlay.

When loaded in the kiln there will be varying amounts of air between the sheets, depending on the glass texture and on how even and true are the mating faces. The air will expand as the firing progresses and some of it will be expelled. Settling of the top sheet as it softens will push out more.

Fig 24-01

When fusing two pieces of float with minimum surface unevenness to trap air, heating can proceed straight from transition to soak at the kiln’s uncontrolled rate, as in Fig 1.

Fig 24-02

With textured sheet, a hold can be programmed at a temperature where the glass is soft but not quite at the tack fuse stage; giving the top sheet more time to slump and push out the air, before the outer edges stick together and stop it escaping, as in Fig 2.

Of course, placing the smoothest faces in contact will limit the amount of air initially enclosed, as well as allowing it to escape most easily.

In side-fired kilns, the edges can often heat faster than the centre, so an intermediate hold is more likely to be needed than in top fired kilns where the glass is heated more evenly across its surface. One technique sometimes used is to insert fine slivers of compatible glass between the sheets so that the edges are held slightly apart. The slivers will eventually be absorbed into the mass, but using this technique is best tried first on a piece where edge development is not crucial.

Where the edges will be visible and the best possible finish is desired, much more care will be needed when fusing than is needed when slumping a single piece of the same total thickness. Most commonly this will be two pieces of 3mm which will be fused together to make a 6mm piece.

Fig 3

Fig 4

In a typical firing at 780°C (1440°F) for 30 minutes in a top-hat kiln, a 6mm piece developed nicely rounded edges as in Fig 3. Whilst having a good body fuse, the edges of two 3mm pieces were behaving as single pieces of 3mm will behave; top edge of the top layer was rolling up-and-over, whilst the bottom of the lower layer had serrated edges and traces of needlepoints, as in Fig 4.

Increasing the soak temperature by 20°C  (35°F)  to 800°C  (1475°F ) caused a rolling-over and thickening of the top edge of the 6mm piece. Surface tension was pulling it over. On the other hand, two 3mm pieces fused together showed good edge development under the same conditions.

When comparing a slump and a fuse of the same total thickness, extra heatwork will be required with the fuse to achieve a similar edge formation. In the case of float it appears to be an increase of about 20°C  (35°F)  in soak temperature.

Fig 5A

Fig 5B

When fusing together two pieces of 3mm float of the same size, with the edges in line as in Fig 5A the edges will flow together but with a slope as in Fig 5B.

Fig 6A

Fig 6B

However, when the top layer is cut about 1mm larger than the bottom, so that the top piece overlaps the bottom by about half a millimeter, the fired edges will be nicely rounded but with the axis vertical, as in Fig 6B.

A similar situation prevails when fusing thicker pieces together. Edges will have better shape if top layer slightly overhangs the bottom layer.

Fusing small piece on a larger piece

Fig 7

This is an arrangement usually used when testing for compatibility, but is also used frequently in general work.

Fig 8

Fig 9

When fired, the small piece can sit almost proud of the base piece, as in Fig 8, or be sunk into the base layer as in  Fig 9. When the top piece is barely protruding above the base glass it is referred to as a flat fuse. Considerable extra heatwork is required to progress from a simple fuse to a flat one.

All the glass must become more fluid; the base layer so it can flow sideways to ‘get-out-of-the-way’ to accept the patch and the top piece to also flow sideways to level out with its surrounds.

The top piece can be sizeable or it can be a shard or piece of stringer. The amount of heatwork needed can vary considerably from one to another.

A simple fuse is adequate if testing for compatibility. There must be full bonding of the two pieces at the edges but deep penetration into the base layer is not required.

Segmented layer with single piece layer

Fig 10

One full size layer in this arrangement is combined with a second layer comprising a number of pieces making up a design. These segments might be of contrasting colours; a multi colour design layer fused to a compatible clear layer is common.

The layer made up of multiple pieces can be placed on top of the base layer, as in Fig 10, or the layers can be reversed so that the single piece layer is on top.

Fig 11

Fig 12

These involve both a horizontal fuse between the layers and vertical fuses between the pieces comprising the segmented layer.

Unfortunately, gravity doesn’t work sideways, so the glass must flow downwards to fill any voids from the bottom up. If the edges of the segments are square and close fitting they will fuse and flow together with little discernible irregularity of the surface.

If, however, there are gaps between the faces, as in Fig 11, then the glass must flow downwards and sideways to fill the gap. Depending on the width of the original gap, there can be a dip in the surface as in Fig 12, which considerable additional heatwork may or may not be able to level up.

Fig 13

Wide or uneven gaps as in Fig 13 should be avoided as they necessitate excessive amounts of heatwork to achieve an acceptable join. This is less of a problem with straight cuts than with curves. Accurate cutting, or cleaning up on a spindle grinder or sander can minimise these problems. 

Fig 14

Rolled glass can vary somewhat in thickness, so that the top surfaces of adjoining segments can be uneven, as in Fig 14. It makes good sense to keep this variation to a minimum, but it often cannot be avoided.

As discussed elsewhere, glass of different colours absorb heat at different rates, with dark colours heating faster than light or clear pieces.

Fig 15

Where the difference in height is significant, and particularly where there is wide colour contrast between them, then the flow of one colour onto the other can sometimes cause the finished join to appear wavy or uneven, as in Fig 15.

Fig 16

When segments are narrow where they reach an edge of a piece, as in Fig 16, difference in thickness or the way they flow can often produce uneven edges.

Fig 17

This can be minimised by incorporating into the design a border to edges where there are many joins, as in Fig 17.

A strip of the glass ‘A’ from the design can be fitted to the bottom edge so that the narrow strips can be butted up to it and the total number of joins at the edges is reduced. The join where the two pieces ‘A’ meet will be a joining together of like glass, thickness will be the same and conditions will be best to achieve a level and smooth edge.

	Fig 18: 2098 Deirdre Feeney, 2007, Cold worked, kiln worked & constructed glass, 135mm x 195mm x 135mm
	Detail of bricks

Of course, there are times when it is intended that the joins should show, and loss of definition would detract from the piece. The tiny ‘bricks’ in Fig 18 would not be bricks if the joins between the mating pieces were to level out and disappear.

Now lets consider the alternative arrangement to Fig 10, where the single piece of glass is on the top and the segmented layer is on the bottom.

Here, the segments must be placed directly onto the supporting surface.The risk of foreign matter becoming trapped between the mating faces of the segments generally precludes the use of anything other than a smooth battwashed kiln shelf or a new sheet of CF paper as the supporting surface.

The gaps between mating faces must be kept to a minimum, in this case because of the risk of bubbles forming if air is trapped.

Among reasons cited for using this arrangement are that when a clear piece is placed over the coloured base layer the resulting joins will appear straighter, the perception of the colours will be different from when they are on the top, and the top surface will be smoother.

Against this, should the bottom layer not be of uniform thickness, then air trapped in pockets by a thick piece could also encourage bubble formation if it cannot all escape.

A variant on this is where both layers are segmented. The comment above would still apply to the bottom layer. This may possibly be done where one lacks a single piece to make up the bottom layer so must make it from two or more pieces. The fewer pieces making up the bottom layer the better.

For optimum strength in the fired piece the joins in the two layers should not coincide but should be at an angle to one another.

Single segmented layer

In this arrangement two or more pieces may be laid side-by-side with edges touching in a single layer.

This is most likely done with rolled glass to achieve colour contrast. It is not often done with larger pieces as large areas of 3mm glass would be fragile.

It is more frequently done when making up small hanging objects from scrap pieces, or when making novelties

It may occasionally occur by accident when two pieces are placed too close together in a kiln and can flow and fuse, or maybe the Kiln God moves them around!

The same problems as mentioned previously can occur should foreign matter get between the faces of the joins or should they not fit closely.

Other techniques

It is good practice to keep to a minimum the number of firings to which a piece of glass is subject. Should a complex piece comprise many layers one on top of another, then adding them one-at-time can only increase the possibility of problems with the most fired layers. Better to build up in a series of sub-assemblies with a final assembly firing to put them all together. 

To create sculptured surfaces or three dimensional effects, smaller pieces of glass may be fused between larger pieces.

Often, lead bearing  paint is entrapped between glass layers to make it food safe. Most times this will be onto float glass.

As explained elsewhere, applying paint to the tin side can cause change in the colour.

The ‘caution’ about not applying the paint to the tin side is difficult to follow here as the paint must come in contact with the tin if the other ‘caution’ of firing with tin side down to minimise tin bloom is to be followed.