Cast Iron Mould Risks

There is a lot of concern about the safety of many products used in kilnforming, and much of it is based on hearsay. The best source for understanding the health and safety risks is Gregorie Rawls website.

Another, but more difficult to interpret, source is the SDS for each product.

Cast iron composition and safety

In this case the investigation is cast iron used as moulds. The first element is to know what cast iron is:

Cast iron is a class of iron-carbon alloys having a carbon content of more than 2% and silicon content around 1–3% with a melting point of 1,539°C (2,802°F). [Wikipedia]

The SDS gives the following information on Gray Cast Iron, the material of the cookware commonly used in the kiln:

• This material is rated as NOT HAZARDOUS by OSHA

• Appearance and Odor: Solid Mass, No Odor

• Specific Gravity: 7.86

• Boiling Point: 5000F

• 5 mg/m³ is the Time Weighted Average (TWA*) for fumes over an eight hour day. https://www.cdc.gov/niosh/idlh/1309371.html

These indicate there is no risk from fumes during casting firings as melting point will not be reached and the boiling point of cast iron is much higher than kilnforming kilns can reach.

The real risks are at room temperature, and are from the powder that may be created while grinding or smoothing the metal surface. The TWA* for cast iron dust is 10 mg/m³ over 8 hours. There are two alloy elements that also may be of concern – nickel and chromium. The amounts are low – chrome is from 0.5% to 2.5%, and even less nickel. The amounts are very low, giving little possible exposure.

The health concerns about using cast iron as a mould seems to be one of the misapprehensions of the amount of exposure, and therefore risk, that are common. The precautions are to have ventilation at source, use eye protection, and wear a N95 respirator.

The use of cast iron as a mould material

“Cast iron is a poor heat conductor compared to copper and aluminium, and this can result in uneven heating if a cast-iron pan is heated too quickly or… [unevenly].  Cast iron …[is] capable of storing more heat longer than... stainless steel pans. Slow heating... can lead to a more even temperature distribution. Due to the thermal mass of cast-iron utensils… they can retain heat for a long time...” Wikipedia.

This indicates that slower than usual ramp rates are advisable during the heat up to avoid breaking the cast iron through uneven heating.

Another thing to note is that the expansion rate [CoE] of between 106 and 114. The mould will contract more than glass, so preparing the mould with smooth sides and a sufficient draft is important to being able to remove the glass from the mould.

*Time Weighted Average (TWA) example:

"Rarely is exposure consistent throughout the day. Let’s say you are working in your studio for 8 hours grinding glass and exposure varies throughout the day… [Exposure varies in amounts]. The exposures throughout the day are averaged and the Time Weighted Average is determined. [In the example cited], … the OEL = 10 mg/m3 and the Time Weighted Average is 3.2 mg/m3, so actual exposure is below OEL (Occupational Exposure Limit)."    https://gregorieglass.com/general-information

Elevation of Moulds

Is it necessary to elevate slumping moulds above the shelf? 

I first heard of the need to elevate moulds from a Bhole representative about 2007. I ignored it, but didn't get around to testing until working on my e-book Low Temperature Kilnforming.

That work showed there is a larger difference in air temperature above and below the unsupported mould than the supported one. But that difference is much smaller than between the air temperature and the glass.

At 150°C/270°F per hour the maximum difference in the temperature under the mould between the elevated and on-the-shelf mould at top temperature was 41°C/74°F while the air temperature difference was 126°C/227°F higher than under the elevated mould.  Many of the tests showed less difference than the maximums given here.

By reducing the ramp rate from 150°C/270°F per hour to 120°C/216°F, the under mould to above mould differential was reduced by a quarter. I didn't test beyond that. But it would appear that slower rates of 100°C/180°F and less will reduce that differential.

The graph also shows that there is a large difference between what the pyrometer reads than the mould temperature of the slump. Slower ramp rates produce an air temperature much closer to the mould temperatures.

Shortly into the rapid cool towards anneal soak and cool only minor temperature difference showed between elevated and on-the-shelf moulds throughout the anneal soak and anneal cool.

These details make it clear to me that elevating moulds is completely unnecessary with slow ramp rates. This of course, fits with the low and slow mantra that many of us promote. However elevating the mould will not harm the slump.

One caution, though. Damp. Wet, or heavy moulds must be supported to avoid breaking the shelf. So I advocate placing these moulds on the floor of the kiln with 2cm posts, rather than on the shelf. I don't know if it is necessary. I haven't tested it. But I do know that moulds in this condition will break the shelf without significant separation between the two.

Low Temperature Kilnforming e-book is available from Bullseye  and Etsy and is applicable to all fusing glasses.

Using Ceramic to Drape

Characteristics

Before choosing a ceramic shape to use in draping of glass, you need to consider the characteristics of the two materials.  This is one circumstance where CoE is actually useful. 

The expansion of the two materials is different. 

• Soda lime glass typically has an expansion rate - in the 0°C to 300°C range - of 81 to 104.  
• Ceramic has an expansion rate - in the 0°C to 400°C range - of 30 to 64.  

This is important in the final cooling of the project.  As the glass expands more than the ceramic on the heat-up, so it also contracts more during the cool.  This means that the glass will shrink enough to trap the ceramic or even break if the stress on the glass is too much. 

Shape

The shape of the ceramic form will have a big effect on the usability of it as a mould.  Ceramics with right angles between the flat surface and the sides will not be suitable for draping without modifications or cushioning.  The forms suitable for draping need to have a significant draft to work well.

Ceramic forms such as rectangles, cubes, and cylinders do not have any draft in their form.  

A cube shape unsuitable for draping

Ceramic cylinders with straight sides

Although rounded at the base, the sides are too straight to be a draping mould

The glass will contract around these forms until they are stuck to the ceramic or break from the force of the contraction around the ceramic.

You can experience this trapping effect in a stack of drinking glasses.  Sometimes one glass sticks inside another even though there is a slope (i.e., a draft) on the sides of the glasses. This happens mostly when you put a cold glass inside a warm one.  On cooling the warm glass contracts to trap the cooler one. You can separate these by running hot water on the bottom glass, so that it expands and releases the inner, now cool, one. 

Effect of Shape

The ceramic contracts at about half the rate the glass contracts (on average), unlike steel which contracts faster than the glass. This means steel contracts away from the glass, while the glass contracts against the ceramic, on the cooling.

Because the glass is in its brittle or solid phase during the last 300°C to 400°C, this contraction tightens the glass against the ceramic, causing stress in the glass, even to the point of breaking.

However, if you choose ceramic forms with significant draft, you can drape over ceramic.  This is possible when the slope is great enough and the form is coated with enough separator, to allow the glass to slip upwards as it contracts more than the form. Experience with different draft forms will give you a feel for the degree of slope required. 

 

These pyramid shapes have sufficient draft to allow the glass to move up the mould during cooling.

Compensation for Lack of Draft

You can compensate for the insufficient draft of ceramic forms by increasing the thickness of the separators for the form.  The hot glass will conform to the hot ceramic, so there needs to be a means of keeping the glass from compressing the form while cooling.  This can most easily be done by wrapping the form that has little or no draft with 3mm ceramic fibre paper.  It is possible to get by with as little as 1mm fibre paper, but I like the assurance of the thicker material.

Kiln post wrapped in 3mm fibre paper with cap over the post's hole.

The fibre paper can be held to the form by thin wire wrapped around the outside of the fibre paper. The advantage of the 3mm fibre paper is that the wire will sink below the surface of the paper.  You can tie off the wire with a couple of twists.  Cut off the ends and push the twist flat to the fibre paper to keep the glass from catching onto the wire.  If you want further assurance, you can put a bit of kiln wash onto the wire.

Conclusion

The choice of ceramic shapes to drape glass over is very important.  It needs to have sufficient draft and separator to allow the glass to slip upwards as it contracts more than the ceramic during the cooling.  You often can use items with no draft if you wrap fibre paper around the sides of the form.

Reversibility of Boron Nitride

After using Zyp/MR97, can I sand it off and use kiln wash?

Some people are applying boron nitride to ceramic moulds for the "smoother" surface.  Boron nitride is an excellent separator for metal moulds and casting moulds whether metal or ceramic. But it has limitations, including the price and requirement for a repeated application at each firing.  Some are beginning to wonder if they can go back to kiln wash after having used the boron nitride. Some say you cannot unless you sand off the separator.

The general experience has been that you can't apply kiln wash on top of the boron nitride. It just beads up and flows off, because the boron nitride creates a non-wetting surface that survives relatively high temperatures.  The water in the kiln wash mixture merely beads up or washes away. This means the kiln wash in suspension has no opportunity to adhere to the mould.

The most accepted way to get rid of the boron nitride is by sandblasting. Then apply kiln wash as normal. The sandblasted ceramic mould previously coated accepts kiln wash with no difficulty. In the absence of a sandblaster, you can use a sanding pad. You do need to be cautious about taking the surface of the mould when using abrasive removal methods, as the ceramic is relatively soft in relation to the abrasive materials.

However, boron nitride is soluble in various alkaline chemicals, such as potassium hydroxide (caustic potash), sodium hydroxide (caustic soda or lye), and sodium nitrate (Chile saltpeter).  Soaking or washing the surface with one of these will dissolve the boron nitride, and should return a surface that will accept kiln wash.  Be cautious in the use of these chemicals as they are dangerous to the skin.

The difficulty of removal of the boron nitride means that you have to think carefully about which moulds you put it onto.  If the mould has delicate or fine detail, removing the boron nitride risks the removal of some of the detail.  This indicates that this kind of mould, once coated, should not be taken back to the bare mould to change the kind of separator.

Another use of boron nitride is to spray a very small amount on a fiber strip to be used for damming. This will give you fewer needles as it provides a non-wetting surface at relatively high temperatures. This allows the glass to slide down the fibre paper without hanging up and creating the needles.

One advantage of kiln wash over boron nitride is that you do not have to reapply every firing as with boron nitride. With the boron nitride it is recommended to apply before every firing.  It is best to use a paint brush to dispose of any lose material before giving a light re-coating. Not a whole lot is required on subsequent coatings.

If you are using boron nitride to get a smoother surface to the object, also consider using a lower slumping or draping temperature, as this will also minimise mould marks.  

Hard Spots in Moulds

Hand pouring of slip into a mould

Some ceramic moulds have small areas where the kiln wash does not seem to adhere as well as on the rest of the mould.  This comes from the manufacturing of these slip cast moulds and this blog post explains how it occurs.  The question is what to do to make the mould separate from the glass after firing.

Coat the mould as usual, which shows up the area where it seems no kiln wash is sticking.  There is some coating the area, but not in the same amount as the rest of the mould.  You can add a little extra kiln wash to the area once first layer has dried, but be careful to avoid creating a ridge against the rest of the kiln wash. If one does appear gentle smoothing with a finger can disguise the transition.

Another approach is to abrade the spot a little to make a more textured surface for the kiln wash to attach.  This needs to be done carefully and by hand to avoid creating a shallow divot in the mould.

The safe approach is to coat as usual and slump a sacrificial piece of glass to ensure the glass does not stick to the hard spot.  If it does not, the spot has enough separator to be useable, although I would continue to add kiln wash to that spot for several firings.

 

Specific Gravity of Unknown Glass

(warning: lots of arithmetic)

Knowing the specific gravity of a glass can be useful in calculating the required amount of glass needed, e.g., for casting, and screen and pot melts, where a specific volume needs to be filled.

Most soda lime glass – the stuff kilnformers normally use – is known to have a specific gravity of approximately 2.5.  That is, one cubic centimetre of glass weighs 2.5 grams. 

If you have glass that is of unknown composition for your casting, you will need to calculate it.

Calculating the specific gravity of unknown glass.

Specific gravity is defined as the ratio of the weight of a substance to the weight of water (in simple terms).  This means first weighing the item in grams.  Then you need to find the volume.

Calculating the specific gravity of regularly shaped items

For regularly shaped item this is a matter of measuring length, width and depth in centimetres and multiplying them together. This gives you the volume in cubic centimetres (cc).

As one cubic centimetre of water weighs one gram, these measurements give you equivalence of measurements creating the opportunity to directly calculate weight from volume. To calculate the specific gravity, divide the weight in grams by the volume in cubic centimetres.

An example:

To find the specific gravity of a piece of glass 30cm square and 6mm thick, multiply 30 x 30 x 0.6 = 540cc.  Next weigh the piece of glass. Say it is 1355 grams, so divide 1355gm by 540cc = s.g. of 2.509, but 2.5 is close enough.

Calculating specific gravity for irregularly shaped objects.

The unknown glass is not always regular in dimensions, so another method is required to find the volume.  You still need to weigh the object in grams.

Then put enough water in a measuring vessel, that is marked in cubic centimetres, to cover the object.  Record the volume of water before putting the glass in.  Place the object into the water and record the new volume.  The difference between the two measurements is the volume of the submerged object.  Proceed to divide the weight by the volume as for regularly shaped objects.

Credit: study.com

Application of specific gravity to casting and melts.

To find the amount of glass needed to fill a regularly shaped area to a pre-determined depth, you reverse the formula.  Instead of volume/weight=specific gravity, you multiply the calculated volume of the space by the specific gravity.

The formulas are:

v/w = sg to determine the specific gravity of the glass;

v*sg = w to determine the weight required to fill a volume with the glass.

Where v = volume; w = weight; sg = specific gravity.

You determine the volume or regular shapes by deciding how thick you want the glass to be (in cm) and multiply that by the volume (in cc). 

For rectangles

volume = thickness * length * depth (all in cm)

For circles

Volume = radius * radius * 3.14 (ϖ) * thickness (all in cm)

For ovals

Volume = major radius * minor radius * 3.14 (ϖ) * thickness (all in cm)

Once you have the volume you multiply by the specific gravity to get the weight of glass to be added.

Calculating weight for irregularly shaped moulds.

If the volume to be filled is irregular, you need to find another way to determine the volume.  If your mould will hold water without absorbing it, you can fill the mould using the following method.

Wet fill

Fill the measuring vessel marked in cc to a determined level.  Record that measurement.  Then carefully pour water into the mould until it is full.  Record the resulting amount of water. Subtract the new amount from the starting amount and you have the volume in cubic centimetres which can then be plugged into the formula.

Dry fill

If the mould absorbs water or simply won’t contain it, then you need something that is dry.  Using fine glass frit will give an approximation of the volume.  Fill the mould to the height you want it to be.  Carefully pour, or in some other way move the frit, to a finely graduated measuring vessel that gives cc measurements.  Note the volume and multiply by the specific gravity.  Using the weight of the frit will not give you an accurate measurement of the weight required because of all the air between the particles.

An alternative is to use your powdered kiln wash and proceed in the same way as with frit.  Scrape any excess powder off the mould.  Do not compact the powder. And be careful to avoid compacting the powder as you pour it into the measuring vessel.  If you compact it, it will not have the same volume as when it was in the mould.  It will be less, and so you will underestimate the volume and therefore the weight of glass required.

Irregular mould frames

If you have an irregular mould frame such as those used for pot and screen melts that you do not want to completely fill, you need to do an additional calculation.  First measure the height of the frame and record it.  Fill and level the frame with kiln wash or fine frit.  Do not compact it.  Carefully transfer the material to the measuring vessel and record the volume in cc.

Calculate the weight in grams required to fill the mould to the top using the specific gravity.  Determine what thickness you want the glass to be.  Divide that by the total height of the mould frame (all in cm) to give the proportion of the frame you want to fill.  Multiply that fraction times the weight required to fill the whole frame to the top.

E.g. The filled frame would require 2500 gms of glass.  The frame is 2 cm high, but you want the glass to be 0.6 high.  Divide 0.6 by 2 to get 0.3.  Multiply that by 2500 to get 750 grams required.

Regular mould frames

For a regular shaped mould, you can do the whole process by calculations.  Find the volume, multiply by specific gravity to get the weight for a full mould.  Measure the height (in cm) of the mould frame and use that to divide into the desired level of fill (in cm).

E.g. The weight required is volume * specific gravity * final height/ height of the mould.

The maths required is simple once you have the formulae in mind.  All measured in centimetres and cubic centimetres

Essential formulae for calculating the weight of glass required to fill moulds (all measurements in cm.):

Volume of a rectangle = thickness*length*width

Volume of a circle = radius squared (radius*radius) * ϖ (3.14) * thickness

Volume of an oval = long radius * short radius * ϖ (3.14) * thickness

Specific gravity = volume/ weight

Revised 18.1.25

Glass Volume for a Frit Mould

There are several ways to determine the volume of a mould. 

Calculation of the weight of glass needed

Calculate the amount in the metric system of measures, as that gives much easier calculations. Cubic centimetres of volume times the specific gravity of glass (2.5) will give you the number of grams of glass required.

This works best on regular geometric forms.  Rectangles and parallelograms are easy to measure the length, width and depth in centimetres.  Multiply together and you obtain cubic centimetres.  That times the specific gravity – 2.5 – will give the number of grams to fill the mould.  The frit will of course be mounded above the levelled surface, because of the air spaces between the frit pieces.

Example of a small frit mould

Irregular shaped moulds

The moulds which are irregular in shape or depth are more difficult to calculate. 

You can determine the volume by starting with a measured amount of water.  Quickly fill the mould to the surface, so that no water is absorbed into the mould. Empty the water from the mould into the drain so it does not become soaked. The difference between the starting and finishing amount of water is the volume of glass required to fill the mould. 

You can use that volume in cubic centimetres times the specific gravity (2.5) to get the number of grams of glass required.

However, it is much easier to put the frit into the water until the measure shows the same amount as before the mould was filled. Then you only need pour off the water and allow glass and mould to dry.  No calculation required.

This post gives some alternatives.

Revised 30.12.24

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.

Mould Elevation

 

The expansion characteristics of glass and ceramics components

Many people advocate the elevation of moulds.  Mainly for air flow to equalise temperatures above and below the mould.  But also, to prolong the life of the mould.  My observation on these reasons for elevating the mould are that they are not harmful, but not necessary, except for investment moulds.

My experiments have showed insignificant differences in temperatures above and below whether elevated or not.  Since the air temperature under the mould is much the same whether elevated or not, it indicates that elevation of the mould has no significant effect.  But, of course, elevation of the mould does no harm either. 

More important than elevation of the mould, is consideration of the nature of the ceramic mould.  Ceramics have two expansion/contraction temperatures called inversions.  The first is at 226˚C/439˚F, and the second around 570˚C/1058˚F.  The ceramic expands rapidly at these temperatures.  There is a 2.5% increase in volume at 226˚C and a slightly more gradual 1% increase around 570˚C.

This a main reason to use slow ramp rates up to at least 570˚C/1058˚F.  Slower rates ease the ceramic expansion speed and reduces the risk of breaking.  So, slower rates will lengthen the life of ceramic moulds. The cool down for annealing and cooling is slow enough that it presents no risk for the ceramic.

There are occasions when the mould must be elevated, though.  These are when the mould is large, heavy, or damp.  This is to protect the shelf rather than the mould or glass.

 

Deep Slumps with Bubbles

 

Photo Credit: Rachel Meadows-Ibrahim

The main causes of the large thin bubble is most probably  too high a temperature combined with a long soak.

Elevation of the Mould

The poster indicated there are eight holes total – four on the sides and four under the glass. This means any air has an exit out from under the glass and from the inside of the mould. So, in this case it does not need to be elevated for exit of air.  In my practice l have never, except in tests, elevated my slumping moulds. I have not had failures. My experiments involved in writing the eBook Low Temperature Kilnforming  showed no significant temperature differences between elevating, or not, below the mould.

Effect of Fast Rates

Slow rates to low temperatures with long soaks avoid sealing the glass to the mould. This means air can move out from under the glass during the slump. 

• Fast rates, and elevated temperatures can restrict air movement from under the slumping glass.  
• Fast rates and high slump temperatures can each cause uprisings because the glass slides down the mould during the soak, and that weight pushes the bottom upwards.

Temperature and Uprisings

This uprising is different from the bubble at the bottom on this piece. It is possible to see the glass bubble is thinner than the surrounding glass. As there were holes for air to escape, it seems the temperature and or speed was great enough to allow the glass to form to the mould at the bottom.  This covered the air holes and allowed the remaining air to push upwards on the glass.  A lower top temperature may have avoided this bubble formation.  Certainly, a combination of a slower rate and a lower temperature would have avoided the formation of the bubble.

Observation

Further, observation during the firing would have caught this bubble formation early enough to skip to the annealing and result in a piece with only a slight uprising, and before it became a bubble. Peeking should start at the beginning of the slumping soak and be repeated at 5 to 10 minute intervals.

Uneven Slumps on Deep Rectangular Moulds

 "Can anyone please tell me why this mould always comes out wonky and devitrifies and pulls in on the edges. I used Primo Primer; my kiln is level, and this is the slump schedule I use for 3mm base with 6mm in places [temperatures in Celsius]: 100/593/30 mins; 66/663/25 mins; 204/482/60 mins; 66/371/10 mins; END.*   12cm square."

 GlassAlisa

The suggestion has been made that having a 6mm base would lessen the irregular slump in the mould.  I am not convinced that making the base thicker will sort the problems.

 

1)  This is a very deep mould in relation to the span.  The mould sides are steep.

 

2) The glass slides down and picks up marks from sliding down the walls of the mould.  The marks are not devitrification.

 

3) Deep slumps are prone to going off centre. One fix is to watch and be prepared to reach in with wet sticks to readjust the glass placement on the mould.  

 

4) Deep moulds (deep is relative to the span of the mould) require two or more stages of slumping. Start with shallow a slump, and progress through steeper ones.

5) The sides dog bone on many rectangular moulds.  One way to reduce this is to round the corners with a 10mm radius.

 

6) Reducing the forming temperature, and extending the soak time dramatically, will go some way to alleviating the previous problems. I suggest trying a 620C slump temperature and soak for 2 - 3 hours.  Peek at intervals to see when the slump is complete, then advance to anneal and cool.

In my view, it is a mould from a maker that does not fully understand glass behaviour.

And in passing, the ramp speed from top temperature to annealing should be as fast as possible, to avoid any risk of devitrification on the way down.  

* Schedule in Fahrenheit for the Americans.  

• 198 to 1100, 30' 
• 119 to 1225, 25'
• 367 to 900, 60'  [ASAP is the recommended rate.  As it is a tack fused piece, I would anneal as though 12mm/4 layers.  This would use a 2 hour soak, cool at 100 to 800, 180 to 700, off ]
• 120 to 700, 10'
• End

Slumping Breaks on “go-to” Schedules

 An "It has always worked for me before" schedule implies a single approach to slumping regardless of differing conditions.  Layup alterations, thickness variations, colour contrasts, mould variations all affect the scheduling.  The schedule for each piece needs to be altered when there are changes from the schedule for the “standard” piece, or mould.

Photo credit: Emma Lee

In the example shown, we are not told the schedule, but it shows that the rate was a little too fast. If it had been faster the glass would have separated further apart. The heat was enough to appear to recombine at the edges where it was not slumping so much. 

Review your "go to" schedules whenever something changes. It may still be a good base from which to work. But you need to assess the layup, thickness, and any other variations to help adjust the schedule to fire each piece.

Some of the variations from the “standard” to be considered are:

Single layer slumping 

Weight

Mould sizes

Relative Slumping Depth

Mould shape and detail