Rigidisers - Application and Use

credit: Scarva

 

Material

Rigidisers are colloidal solutions of silica or quartz with a carrier of some form.  It is also available as a powder to mix with water according to the instructions.

Health and Safety 

Silica and quartz (sometimes referred to as flint) in dry powdered form are a serious health risk.  Wear good respiratory protection and long sleeves and gloves against its skin irritant.  Work outside with the powdered form to keep the dust out of the studio. Clean clothing immediately after working with the powdered form of rigidiser.  Wearing gloves is a good idea whenever working with rigidisers, as the wet form is also a strong skin irritant.

Application

Mix up the powdered form as 1 part powder to 4 parts water, by volume.  Do this masked and gloved, and outdoors if possible.  If not, have a HEPA vacuum running next to your work area.  Mix thoroughly and allow to slake for 24 hours.  Then mix very well by hand or with a blender.  Strain the mix to remove any clumps - they can be made into a paste and added to the main solution.

Liberally paint the solution onto the refractory fibre.  Stir prior to use and frequently throughout the application to keep the silica/quartz in suspension.  Depending on permanence, coat one or both sides of the paper/blanket/board.  It is not necessary to soak the fibre completely.  The object is to provide a hard surface.  It does not need to be hard throughout.

Flat Board

It is best to apply rigidiser on both sides of refractory board.  If rigidising both sides, allow one side to air dry before turning over to coat the other side.  By coating both sides, the warping from heating on one side is reduced. 

Slumping forms 

Cover the shape you are taking the mould from with an impervious separator such as Vaseline or thin plastic film.  Prepare the fibre blanket by coating both sides of the fibre with the rigidiser.  It does not need to be completely soaked.  Press the fibre firmly into/onto the shape and especially into any depressions and around any protrusions to be certain of a faithful replication.

Curing  

Allow the refractory fibre to air dry.  Or if needed quickly, you can kiln dry at 90˚C – 110˚C / 194˚F – 258˚F for several hours.  But only if the master mould can withstand the heat.  If not, demould only after the fibre is dry and can hold its shape without the master.  Be sure to remove the master mould from the fibre before proceeding to heat cure.

When air dried, cure in the kiln by firing to 790˚C/1454˚F for 20 minutes.  Before firing, place the dry form on a refractory fibre separator to avoid the silica/quartz sticking to the shelf. A rapid rate straight to the top temperature is acceptable.  After the soak, turn the kiln off, as the rigidised refractory material is not subject to thermal shock.

In Use

Coat the hardened fibre in kiln wash, or cover with shelf paper or refractory fibre paper, to avoid glass sticking to the hardened board.  The bare surface of the rigidised form is now coated in glass fibres and they will stick to the glass unless a separator is applied.

When used as a shelf, it is best to turn the board over after a few dozen firings. This helps counteract the warping tendency that rigidised boards have.

Fold Moulds

 

These moulds are available in stainless steel forms in various sizes

You can create your own mould for self-supporting display items. Fibre board and vermiculite board are suitable.

 

I chose 25mm/1” fibre board because I had a suitable piece lying around. It is possible to use thinner fibre board, but the thicker board is more likely to resist deformation over a long use period. The 15mm/0.675” board is suitable for light use. These do not need to be rigidised unless you desire to for a more robust structure. They do not need to be kiln washed unless you feel a better surface will be achieved.

Angled Surface

The 25mm/1” vermiculite board is more durable. It does need to be kiln washed to avoid glass sticking to it. Otherwise’ it is treated just the same as the fibre board.

The width and length of the board are determined by the width and length of the piece you are currently making or envisage making. You can make it longer than current needs and use a stop of a piece of fibre board or other kiln furniture to ensure the glass does not slip down the slope. This allows you to adjust the mould to different lengths for a variety of projects.

Both materials need to have an angle cut from one end. This is the end that will be elevated. It allows the glass to bend directly from the end of the angled board. This angle does not need to be more than 30 degrees from vertical, as most self-supporting items have angles of about 15 degrees or less.

Support

Then a support piece needs to be made. If it is not of fibre board, it needs to be kiln washed to prevent the glass from sticking. This support needs to be as wide as the angled board. The height of support will determine the angle of the finished piece.

It needs to be aligned vertically and directly under the top of the angled board. A try square can help with this alignment. This support also stops the draping glass from curving under the top. It would be interesting for a rocking horse kind of item, but not for a stable decoration.

The support under the elevated end can be made to various heights to obtain various angles on the piece. Also, different heights of support will be required to maintain the same angle on different lengths of the standing piece. This makes the home-made mould much more versatile than the steel ones.

The Stop

The stop is a piece of kiln furniture placed on the slope at the end of the glass to ensure the glass does not slip down during the firing. It is not fixed to the sloped board so that it can be repositioned. If you are using fibre board for the slope and the stop, you can pin the stop to the sloped board. Or you can use heavier kiln furniture, propped as appropriate to form the stop.

Firing notes

Glass lengths

The length of base in relation to upright needs to be determined before firing. You can, of course, cut the excess base length off after firing. I make the base to be the same length as the top leans back. This ensures the piece will not become top heavy.

A spirit level can be used to determine how long the support needs to be. You already know how long the sloped piece of glass is. Place the stop at that distance from the top end of the sloped board. Use a spirit level to indicate the length the base will need to be. When levelled, make a mark on the support. Then measure the distance from the mark to the top of the slope. That length plus the length of the sloped glass will equal the total length of the flat glass.

Scheduling

Use a moderate ramp rate for the thickness of the glass. The top temperature should be about 650˚C/1200˚F. Set the soak time for an hour. Peek frequently from the start of the hold to be sure the glass has draped vertically. When it has advance to the next segment and proceed to anneal.

Damming for Exact Shapes

 Many times, exact dimensions of the final piece are not critical.  When they are and the piece is 9mm and thicker, or has irregular amounts of glass near the edge, damming is required.

 If the dimensions are rectangular, you can use straight edged refractory materials, usually sawn up broken kiln shelves, vermiculite, or fibre board strips.  

 These need to be kiln washed and lined with fibre paper.  The dams should be lined with 3mm fibre paper that is 3mm narrower than the final height of the piece.  This allows a bullnose shape at the edge to form.

 If the shape is a circular or irregular shape the dams can be made from thick fibre board or vermiculite.  The lining of the dams is the same as for rectangular shapes.  

 The use of 3mm fibre paper means that you have to make rectangular shapes 6mm bigger in each direction to achieve the exact final dimensions.  For circular or irregular shapes, the edge will need to be only 3mm larger.  This is because the edge goes around the whole shape, rather than only one side.

 

Vitrigraph Pots from Refractories

Many people are now buying or having made stainless steel square pots for doing stringer and murinni pulls.  This may be the best material for the purpose, but square pots can be made from other refractory materials.

Vermiculite

One material that can be used multiple times is 25mm vermuculite board.  These can be cut to a convenient size and fastened together with stainless steel or brass screws.

If you want to make a pot 100mm square and 100mm high, cut four boards 125mm square, and one 100mm square for the bottom.  This last one will need to have a hole of the desired size drilled at its centre.  A 19mm diameter hole is a medium sized hole.  Keep in mind that you cannot make it smaller, but you can make it bigger. 

Place the four larger squares around the base.  Drill pilot holes for the screws (if you don’t you will split the boards).  Two or three holes along each edge should be as much as needed for long term security. 

The screws at the left side of the box are omitted in the drawing, but are required

Alternatively, you can make the base to fit onto the bottom rather than inside.  In this case, cut the base to 150mm square and fix it to the sides with stainless steel screws from the bottom.

The screws at the left side of the box are omitted in the drawing, but are required

A disadvantage of the vermiculite is that glass sticks to it.  You can overcome this by lining the bottom and inside of the square with 2mm or 3mm fibre paper.  You cannot hammer out the residual glass without destroying the whole box. This lining will protect the surfaces, and the fibre can be removed after a firing, leaving clean sides.  This will not be as long lasting as the stainless steel pot is, but it is economical and adaptable to your specific requirements.

Refractory fibre

Another refractory material that can be used to construct square pots is 25mm refractory fibre board.  The sizes of the components are the same as for vermiculite.  This time rather than screws, you need stainless steel pins – 50mm long sewing pins are suitable.  You can also use high temperature wire, about 50mm long with a small right-angle hook at one end.

Assemble the four sides around the bottom as previously, and push the pins into the board to secure them.  You will need more pins than you did screws for the vermiculite.

As an additional securing measure, wrap the box horizontally with two bands of 0.5mm kanthal or high temperature wire and twist the ends together.  Then on two sides wrap more of the high temperature wire under the bottom and twist the ends together on the top of the two sides.  These wires do not have to be really tight.  They are there to prevent any failure of the pins.

This refractory fibre box is light weight although it will not last as long as the vermiculite one, but it is quick and easy to put together with a minimum of tools – knife, wire cutter, straight edge.  You can line this with fibre paper as for the vermiculite.

Safety note:

When working with vermiculite or refractory fibre, you should wear breathing protection and dust your clothing outside or change after the box is complete and put those clothes in the washing machine.

You can make a vitrigraph box from refractory materials rather than buying a stainless steel one.  Information on making one is given.

Kiln Characteristics Investigation

Many people ask about the best kiln to buy.  Sometimes they mean the cheapest, but mostly they mean the best for their favoured processes. To get the best from your proposed kiln, you should be aware of its characteristics and how it fits your proposed kilnforming practice.  There are a range of factors that interact to give the special conditions of your kiln.  They range from the purpose, the materials of construction, the placement of heating elements, how it opens, and its shape.  All these can affect the degree of even heating of the kiln bed or shelf.

Kiln types

There kilns for many purposes. Some of them are powder coating of metals, enameling of metals, vitreous painting of glass, glass forming, ceramics, casting of glass and metals, lehrs for annealing, and furnaces among many others.  

Large powder coating kiln

Large enameling kiln

Jewellery enameling kiln

Electric glass painting kiln with multiple shelves

Example of a sheet glass annealing lehr

For our purposes we are concerned with the glass and ceramics kilns.

Ceramics vs Glass Kilns

In general ceramics kilns are made to lose heat slowly, while glass ones are designed to lose heat relatively quickly.  There are many glass kilns based on ceramic ones.  You should be aware of the differences between kilns designed exclusively for glass and those based on ceramics kiln designs.

Small ceramic kiln

Small glass kiln

Construction Materials 

The materials used in constructing kilns are refractory insulation and a steel structure of a design to hold all the refractory materials together. 

Refractory bricks for glass kilns are light weight and usually designed for temperatures under 1200°C (dense bricks rated much higher are normally used in ceramic kilns). 

Light weight refractory brick

Bricks tend to be used in most glass kilns on the floor as well as the walls (some smaller ones use only refractory fibre).

Small fibre kiln

Kilns derived from ceramics tend to have brick walls and lids.  Most kilns designed for kilnforming have fibre walls and lids.  In the cases of top hat opening kilns, fibre is a necessity to reduce the weight of the lid.

Fibre board and fibre blanket are used widely.  The floor tends to have a floor consisting of steel, fibre board on top and brick on top of the board. Fibre blanket tends to be used on the walls and ceilings of rectangular glass kilns. Oval and circular ones tend to have brick walls and ceilings.  The use of fibre board and blanket walls and ceilings leads to a more rapid cooling than those with brick ones.  This will affect the scheduling of the kiln firings.

The steel used to contain and support the refractory materials is important.  Many kilns use mild steel in sheet form to fill the spaces between the heavier structural support steel.  The higher quality kilns use stainless steel sheet, even though they may use mild steel for structural support.  The stainless steel lasts much longer than mild steel, especially when there is liable to be moisture involved in the kiln processes, such as pate de verre or casting.

Opening Method

This post gives a description of the common methods of opening the kiln.  

The purposes for which you want to use the kiln relate to the firing characteristics needed.

Top opening

Top opening kilns have the advantage of depth, normally with elements around the sides.  This makes them good for casting, but not so good for processes that need observation or manipulation.  The depth is most useful in casting  and deep slumping work, but requires a lot of experimentation to make use of multiple shelves in one firing.

Front opening kilns have the advantage of being able to observe the whole depth of the firing, if you protect yourself from the heat that will be dumped from the kiln.  They often have elements on the sides which is an advantage for drops and melts (when observation is necessary).

Top hat opening kilns are those that have the whole heating chamber hinged at the shelf level.  These are very good for placing of work, as you can work directly above the pieces.  These are one of the best types of kiln for combing or any other manipulation of the glass during the firing. You can also observe by opening the kiln a little during the firing.

A range of top hat and a bell kiln

Bell kilns are those where the whole of the heating chamber lifts above the bed.  These are often equipped with two bases which can be wheeled in turn under the chamber which is lowered before firing.  These tend to be very large kilns.

Small gas fired kiln

Heat source

Most kilns are heated with electrically powered elements, either exposed or in quartz tubes.  The quartz tube contained elements provide more even heating than the exposed ones.  The most even heat is provided in gas fired kilns, although these are generally more expensive and less widely available.

Element Placing  

The location of the heating elements can have a significant influence on the way you fire your glass.

·        Top fired kilns are generally the easiest to use as the glass is most affected by radiant heat.

·        Side fired kilns provide the radiant heat to the edges of the glass first, before the air temperature can begin to affect the surface of the glass.  This means more caution is required in the heat up of the glass.  However, side elements are very useful in drops and casting processes.

·        Some kilns have both top and side heating elements.  This provides flexibility in heating up and in cooling evenly.

·        A few kilns have elements around the sides but below the shelf.  This promotes even cooling of glass from both the top and bottom. It is most useful in dealing with the cooling of thick slabs.

Kiln sizes and shapes

Kiln sizes have an effect on the behaviour of the kiln.  Smaller kilns (depending on the refractory materials) generally heat and cool quicker than large ones.  The mass of a larger kiln takes more energy to heat up and more time to release the heat than smaller ones do.  This will influence the scheduling for different sized kilns.

 

The shape of the interior of the kiln affects the distribution of heat within the chamber.  Rectangular kilns tend to have cooler corners than circular ones (as there are no corners).  Oval kilns tend to give space for longer pieces and reduce the cool corners.

 

The height of the kiln also affects the heat distribution within the kiln.  Taller kilns are cooler at the bottom than the top, even with side elements.  They are especially good for casting and drop processes.  Deeper kilns, even if rectangular, require more energy to complete any given process, because of the distance between the radiating elements and the glass.

Hot and cold spots can be tested for by using this method.  The actual operating temperatures can be tested by the use Orton cones to measure heat work. This depends on the speed used to get to the process temperature.

There are many factors that make up the characteristics of kilns. The main ones are style, construction materials, opening method, shape and depth. These need to be considered in relation to the kind of kilnforming you intend doing, to make the selection optimum for your practice.

More information is available in "Your New kiln" from Etsy shop VerrierStudio: https://www.etsy.com/uk/shop/VerrierStudio

or direct from stephen.richard43@gmail.com

Materials for making dams

Rectangular or straight sided shapes

Broken shelves

Accidents happen to mullite shelves causing breaks or cracks.  Rather than throwing them out, you can cut them into rectangles or 50mm strips with a tile saw.  The resulting shapes need to be kiln washed to keep glass from sticking.  They can be used flat or stood on their edges with supports on the outside.

Thick ceramic tiles can be used in much the same way.  You do need to remove the glaze from the tile to make sure they don’t stick to the glass.  Or you could use the unglazed side toward the glass. Again, the tiles need to be kiln washed.

Stainless steel can be used as a dam.  It will need treatment with a separator such as boron nitride or kiln wash.  In addition, it needs to be lined with refractory fibre paper to cushion the force of the greater contraction of steel than glass.

These materials cannot easily be adjusted in length to fit the size of the glass piece being dammed.  Instead, arrange them in a swastika like formation. 

This photo also shows how shorter lengths can be incorporated to make the whole dam.

Vermiculite board is a refractory material that can be used to form dams by cutting with a wood working saw.  The saw you use to cut the vermiculite will be dulled and only be useful for cutting vermiculite in the future.  Do not use any expensive cutting equipment!  

Credit: Bullseye Glass Company

Refractory fibre board is available in many thicknesses.  It can be cut with craft knives even though it dulls the blades quickly.  The thicker boards can be used without rigidising.  This avoids the need to kiln wash and allows adjustments in length.  If you do rigidise fibre board, you must coat it with a separator such as kiln wash or boron nitride.

Weighted fibre paper can be used.  It is sometimes the quickest and easiest to use, as there normally is a stash of scraps around the studio.  It is easily cut with a craft knife.  You can build up the thickness of the dam by layering pieces on top of one another.  Sometimes people put metal wire or pins in the layers to ensure there is no movement between the layers. I’ve found that if weighted, the fibres interlock enough that the layers do not shift.  But you need to line the layered fibre paper dams with vertical strips of fibre paper, so the glass does not take up the layered dam profile on its edge.

Note that you need to use breathing protection when cutting all these materials.

Curved and circular pieces

Many times, the shape to be dammed is not formed of straight lines.  Different materials need to be used in these cases.

Formed stainless steel is a good durable and reusable material.  You need to line the shape with fibre paper if it completely contains the shape, because it contracts more than the glass and can crush the piece.  It is expensive to have made and so needs to have multiple uses to justify the cost.  A cheaper alternative is to make your own shape using stainless steel strapping as used for shipping crates and pallets. 

Fibre paper is an excellent material for damming irregular shapes.  It can be cut into complicated shapes, and it can be layered to attain the required height. You can weight it if you are taking things to a high temperature and fear that the glass will flow under the fibre paper.

You can also use the thicker fibre papers upright by backing up with multiple pieces of kiln furniture to maintain the shape you desire.

Vermiculite board is a good material for making shapes, although not as complicated ones as possible in fibre paper.  Vermiculite can be shaped with wood working materials, but cheap ones should be used as they are quickly dulled. You can rough out a shape with a jigsaw and refine it with various wood working tools, including coarse sandpaper.  Because it is a relatively rigid material, a lot of inventiveness can be used in forming the edges by altering angles from the vertical, incising designs into the edge, etc.  Be certain that you have adequately kiln washed or put other separator on the board, as it will stick to the glass if left bare.

Fibre board is a less rigid material than vermiculite, but is easier to work with simple craft tools.  It is simple to use for a unique one-off shape. It only needs smoothing and does not have to have a separator applied because it does not stick to the glass.  If you create a shape that you will want re-use, you can rigidise the board after shaping, but it will require separators then.

Note that when working with refractory materials, you need to wear respiratory protection and clean surfaces with a HEPA vacuum or by dampening dusty surfaces and wiping them clean.  Dispose of cleaning materials safely.

Effects of Dams on Scheduling

 I recently made a statement about the effects of various dam materials on the scheduling.  This was based on my understanding of the density of three common refractory materials used in kilnforming – ceramic shelves, vermiculite board and fibre board.  I decided to test these statements.  I found I was wrong.

I set up a test of the heat gain and loss of the three materials.  This was done without any glass involved to eliminate the influence of the glass on the behaviour of the dams.  The dam materials were laid on the kiln shelf with thermocouples between.  These were connected to a data logger to record the temperatures.

 

The schedule used was a slightly modified one for 6mm:

300°C/hr to 800°C for 10 minutes

Full to 482°C for 60 minutes

83°C to 427, no soak

150°C to 370°C, no soak

400°C to 100°C, end

 

The data retrieved from the data recording is shown by the following graphs.

 

Highlights:

·        The dam materials all perform similarly. 

·        This graph shows the dams have significant differences from the air temperature – up to 190°C – during the first ramp of 300°C/hr. (in this case). 

·        There is the curious fall in the dams’ temperatures during the anneal soak.  This was replicated in additional tests.  I do not currently know the reasons for this.

·        The dams remain cooler than the air temperature until midway during the second cool when (in this kiln) the natural cooling rate takes over.

·        From the second cool to the finish, the dams remain hotter than the air temperature.

 

Some more information is given by looking at the temperature differentials (ΔT) between the materials and the air.  This graph is to assist in investigating how significantly different the materials are. 

This graph is initially confusing as positive numbers indicate the temperature is cooler than the material being compared and hotter with negative numbers.

 

As an assistance to relating the ΔT to the air temperature some relevant data points are given.  The data points relate to the numbers running along the bottom of the graph.

Data Point   Event

    1                Start of anneal soak.

    30              Start of 1^st cool (482°C)

    45              Start of 2^nd cool (427°C)

    65              Start of final cool (370°C)

    89              1^st 55°C of final cool (315°C)

    306             100°C

 

At the data points:

·        At the start of anneal soak the ΔT between the dams is 16°C with the ceramic shelf temperature being 18°C hotter than the air.

·        At the end of the anneal soak of an hour, the air temperature is 20°C higher, although the ΔT between the dams has reduced to 12°C.

·        At the end of the 1^st cool the ΔT between the dams has reduced to 9°C and the ΔT with the air is 3°C.

·        At approximately 450°C the air temperature becomes less than the dams. 

·        At 370°C the hottest dams are approximately 17°C hotter than the air.  The ΔT between the dams is 10°C.

 

More generally:

·        The air temperature tends to be between 17°C hotter and 17°C cooler than the ceramic dams during the anneal soak and cool.  The difference gradually decreases to around 8°C at about 120°C.

·        Ceramic and fibre dams loose heat after annealing at similar rates – generally having a ΔT between 4°C and 1°C, with a peak difference of 9°C at the start of the second cool. This means the heat retention characteristics of ceramic strips and fibre board are very close.

·        Between the annealing soak and about 300°C the vermiculite is between 12°C and 9°C hotter than the same thickness of fibre.  Vermiculite both gains and loses heat more slowly than the ceramic or fibre dams do.  This means that vermiculite is the most heat retentive of the three materials.

Conclusions

·        Dams will have little effect during the heat up of open face dammed glass.  The slight difference will be at the interface of the glass and the dams where there will be a slight cooling effect on the glass.  Therefore, a slightly longer top soak or a slightly higher top temperature may be useful.

·        The continued fall in the dams’ temperature during the anneal soak indicates that this soak should be extended to ensure heat is not being drained from the glass by the dams to give unequal temperatures across the glass with the risk of inadequate annealing.  I suggest the soak should be extended to that for glass of 6mm thicker than actual to account for this.

·        The ability of ceramic and fibre dams to absorb and dissipate heat more quickly indicates that they are better materials for dams than vermiculite board.  The slightly better retention of heat at the annealing soak, indicates that ceramic is a good choice when annealing is critical.

Scheduling Effects 

Based on these observations, I have come to some conclusions about the effect of dams on scheduling.

·        There is no significant effect caused by dams during the heat up, so scheduling of the heat up can be as for the thickness of the glass.

·        The lag in temperature rise by the dams indicates a slightly longer soak at the top temperature (with a minor risk of devitrification), or a higher temperature of, say 10°C can be used.

·        The (strange) continued cooling of the dams during the annealing soak indicates that extending the soak time to that for a piece 6mm thicker than actual is advisable.

·        The cool rates can continue to be as for the actual thickness, as the dam temperatures follow the air temperature with little deviation below the end of the first cool. 

·        Ceramic dams perform the best of the three tested materials.