Gypsum can withstand up to temperatures of around 49°c before it begins to break down. The higher the temperature the quicker the product will break down.
None of the standard range of plasters from BPB formula or Siniat Prestia are designed for high temperatures, so we do not stock a suitable plaster. You need to source a specialist plaster for this specific application.
No plasters are guaranteed outside, as it is not weather resistant. This is because it is porous, thus making it vulnerable to damage from water ingression from the elements such as rain and frost.
However the harder the plaster the longer it will last. Also by mixing a plaster like Crystacal R Plaster with Plaster Polymer instead of water, you are effectively sealing the plaster, so protecting it from the elements that would normally attack and damage the plaster. Another option is to entirely seal the plaster with an exterior sealer or varnish, thus stopping the water ingress.
The closest product to plaster for outside use is probably Jesmonite AC730 or Fibrocem, or other cement based materials.
Prestia Classic is a good plaster to try, it is just that – fine but not too hard to carve. The harder plasters are finer, but obviously can be very hard to carve by hand.
If you are after a polymer reinforced plaster we can offer you 3 options;
Crystacal Lamina Plaster; is a plaster with a powdered acrylic polymer content. So you just use it like a normal plaster and only mix it with water, but it produce a plaster that is much harder and stronger than most plasters.
Plaster Polymer; is a liquid acrylic polymer that you add to plaster instead of water. It is best used with Crystacal R Plaster and mixed 2.5:1, Plaster: Polymer, however it can be used with other plaster and the polymer can also be watered down to make it more cost effective.
Jesmonite AC300; is effectively a polymer reinforced plaster. The AC300 is mixed 2.5:1 Jesmonite Base: Jesmonite AC300 Liquid.
All these products can be pigmented with water based pigments, can be reinforced with fibreglass and scrim, can be retarded and also thickened, for brushable applications.
Any Plaster would be okay for a fireplace, however we would recommend a medium hardness Plaster such as Crystacal R, Herculite No.2 or Prestia Creation. These plasters are more durable and less likely to chip if knocked when installed.
Crystacal Lamina or Crystacast Plaster would be the 2 best plasters for this application. Crystacal Lamina is better as it is slower setting, however it does only come in a 25kg bag. Jesmonite AC300 would also be very good, however it is fast setting, so you may need to use a retarder.
Either a hard Plaster like Crystacast or Jesmonite would be the best for this. You may need to reinforce the cast with fibreglass chopped strand or a fibreglass matt, like Quad-Axial fabric. A porcelain like finish with its smoothness and glossiness on the surface of your cast is going to be somewhat down to the quality and surface of your silicone mould. You can of cause use a clear sealer or varnish (yacht varnish is apparently good) to give the finished surface a shiny, glossy look.
Latex is water based so you can use any water based paint or pigment. You can buy latex pigments that will work best, but we don’t stock them. A water based pigment will work better than paint as the pigment will be stronger in colour.
Tag. Moulding latex, Brushable Latex, Slush Moulding Latex.
The most appropriate Jesmonite product for decorative elements within a wet or external environment would be AC730.
Jesmonite AC730 is a two pack water based, micro-concrete composite material. It is a shrinkage-compensated material of low permeability and is durable in all conditions, including water features.
Please be aware that AC730 is a decorative material and not designed for structural purposes. To create AC730 products which achieve a greater strength to weight ratio, I would recommend they laminate AC730 using x3 layers of Jesmonite Quadaxial Glass with minimum laminate thickness of 12mm.
Once the AC730 is de-moulded and fully cured (demoulded and left for 24 hours) it would then need to be fixed or bonded onto a substructure or frame system.
AC730 contains a blend of decorative aggregates and pigments to achieve a range of stone and concrete effect surface finishes. The AC730 surface will behave in the same way as a concrete or natural stone surface.
1mm thick x 1 metre square requires 2kg mixed material.
Jesmonite is self-supporting when cast about 12mm thick. You would need to strengthen it with 3 layers of Quad-Axial fibreglass matt and possibly laminate in some ‘ribs’ for larger panels. So at 12mm thick you would use 24kg of mixed material per square metre.
Jesmonite AC730 and AC100 aren’t casting composites, they are designed to create lightweight finished products which achieve a greater strength to weight ratio. I would recommend you laminate AC730/AC100 using x3 layers of Jesmonite Quadaxial Glass with recommended laminate thickness of 12mm.
AC730 laminate material coverage:
You will require 2kg of AC730 mixture per mm per M2 of material thickness into your mould.
Formulae: mould size (M2) x 2kg of mixture x mm thickness
Example formulae: 1m2 x 2kg AC730 mixture x 12mm = 24kg total AC730 mixture (please be aware this figure does not take the return edges into account).
AC100 laminate material coverage:
You will require 2kg of AC100 mixture per mm of material thickness into your mould.
Formulae: mould size x 2kg of mixture X mm thickness
Example formulae: 1m2 X 2kg AC100 mixture X 12mm = 24kg total AC100 mixture (please be aware this figure does not take the return edges into account).
Flex Metal Gel Coats coverage: 1-2mm thickness (only compatible with AC730 as a backing material):
Mould with Flat Surface (no texture or pattern): 2kg of Flex Metal Gel Coat Mixture per 1m2 (please be aware this figure does not take the return edges into account).Mould with small texture/pattern: 3kg of Flex Metal Gel Coat Mixture per 1m2 (please be aware this figure does not take the return edges into account).
N.B It is essential to use both accurate scales and a Jesmonite High-shear Mixing Blade to ensure that the compound performs within its specification. Failure to follow these instructions can lead to strength loss, shrinkage, and reduced durability.
Jesmonite Flex Metal coverage is approximately 3kg per mtr square at 2mm thick. This will need to be backed up with AC730 Stone apply 3 layers with Quad-Axial as described above.
Best to apply the first Flex Metal layer then leave it for 30 minutes, then apply the AC730 Stone layers.
The most appropriate Jesmonite product for a fireplace surround would be AC730.
Jesmonite AC730 is a water based micro-concrete composite material. AC730 is U.V stable and contains a blend of decorative aggregates and pigments to achieve a range of stone and concrete effect surface finishes.
We have a number of clients who create fireplace surrounds using Jesmonite AC730 in conjunction with x3 layers of Jesmonite Quadaxial Glass.
Majority of our clients would use a high grade silicone mould for ease of de-mould and fine detail replication. Once the AC730 laminate is cured (4 hour de-mould time) and de-moulded it would then need to be fixed or bonded onto a substructure or frame system.
Jesmonite AC730 isn’t a casting composite it is designed to create lightweight finished products which achieve a greater strength to weight ratio. I would recommend you laminate AC730 using x3 layers of Jesmonite Quadaxial Glass with recommended laminate thickness of 12mm.
However do not expose the material directly to fire/flames.
Please see the below formulae for working out AC730 laminate material coverage:
You will require 2kg of AC730 mixture per mm of material thickness into your mould.
Formulae: mould size X 2kg of mixture X mm thickness
Example formulae: 1m2 X 2kg AC730 mixture X 12mm = 24kg total AC730 mixture.
The most appropriate Jesmonite product to create a smooth polished surface for a master/prototype would be Jesmonite AC100.
Once fully cured the AC100 surface can be finished with sandpaper prior to moulding. For finishing AC100 surfaces smoothly, use a super fine sandpaper with 360- to 600-grit. Once the AC100 has been sanded you could then buff the surface with a white cotton pad to achieve a polished effect.
You could achieve a similar polished surface with AC300, however the finished/cured AC300 product wouldn’t achieve the same strength and durability which you would gain from AC100.
You can achieve a real metallic finish with the Jesmonite Flex Metal Gel Coats used in conjunction with Jesmonite AC730 (including x3 layers of Quadaxial Glass Reinforcement) as a backing material. The Flex Metal Gel Coats use real metal powders and can be polished, burnished, waxed and patinated just like a real metal. The Flex Metal Gel Coat finishes are available in Silver Bronze, Bronze, Brass and Copper.
You can use Jesmonite Flex Metal Gel Coats externally and within wet environments, however the only point to make your client aware of is that due to the Flex Metal Gel Coats containing high levels of real metal powders the surfaces will behave in the exact same way as real metal. The Flex Metal Gel Coat surface will patinate and oxidise naturally overtime.
Jesmonite Flex Metal Gel Coats are a moulded technology and can only applied into negative moulds, they cannot be applied in the positive. Majority of our clients would use a high grade silicone mould for ease of de-mould and fine detail replication.
To polish (minimum 48 hours after de-mould) the Jesmonite Flex Metal Gel Coats you will require polishing wheels. The Flex Silver Bronze, Bronze and Copper will require a Silver polishing wheel, whereas the Flex Brass will require a Brass polishing wheel. Below is a link to for the recommended polishing wheels:
The polishing machine that we recommend is the 6 speed Fein WPO 14-25 E Sander Polisher: http://www.fein-uk.co.uk/en_uk/surface-processing/sanders/wpo-14-25-e-0208131/
Please note that the polishing machine will require an M14 Taper cone thread which attaches to the polishing wheel and polishing machine: https://www.moleroda.com/product/taper-cone-arbor-internal-thread/
The Wire Wool that we recommend is Grade 000: http://www.trollull.co.uk/
Once the Flex Metal Gel Coat surface has cured for a minimum of 48 hours you would then need to polish the surface. Please see the below step by step guide to polishing the Jesmonite Flex Metal Gel Coats.
1. Begin the first step of polishing with the wire wool, use good levels of hand pressure applied up and down the Flex Metal Surface
2. You will then need to set the polishing machine to its highest setting (normally level 6) and rest the weight of the machine onto the Flex Metal surface using a back and forth motion
3. Repeat step 1 with the wire wool
4. You will then need to set polishing machine to its lowest setting (normally level 1) and use higher levels of down wards pressure onto the Flex Metal surface using a back and forth motion
5. Finally repeat step 1 with the wire wool
No, Jesmonite will not stick/laminate direct onto wood. As the wood would move (expand and shrink) with a change temperature, the Jesmonite would delaminate.
The only material it will properly laminate to, other than itself, is Polystyrene.
Stone Fillers can be added to Jesmonite AC100 and AC300. You should use the ratio 2:1 Jesmonite Powder:Jesmonite Liquid (by weight) and then should add the Stone Filler at between 70% and 100% (by weight) to the total mixed Jesmonite. Remember the whiteness of the Jesmonite will make the finish paler than the original Stone Filler, however you can use Jesmonite Pigments to achieve the finish you are after.
You don’t need to use any release agent, so long as the silicone is a reasonable quality material.
Yes you can, although AC730 is primarily a laminating material, you can successfully cast up to about 5kg at a time. If you need to cast larger quantities, you should use AC100, AC300 or AC830.
The best option is to pigment the backing Jesmonite to the closest colour of you metal gelcoat. Brown works well for Bronze for example. It is best to use Jesmonite Pigments, but you could try an acrylic or water based pigment.
It’s recommended to use concealed fixings which are either bonded or laminated into the back of the Jesmonite panels. Jesmonite don’t recommend face fixing Jesmonite panels as they don’t allow for panel and thermal movement and also obscure the decorative surface finish. However the material can be drilled if required.
In regards to fixing and support details for Jesmonite panels we would recommend you contact the below structural engineer:
Smart Crosby International
3 Grosvenor House
Tel: 01572 756 961
Use one of the Jesmonite AC730 Stone materials as the backing material. You will require 2kg of AC730 Stone mixture per mm per M2of material thickness; for example you will need approximately 24kg of total AC730 mixture for a panel 1 M2which is 12mm thick.
This is the statement from the manufacturers;
I can confirm that all Jesmonite materials are non-solvent and contain zero VOC’s. Please be aware that we have never tested any Jesmonite products for food safety.
We do get asked about Jesmonite materials and food safety now and again, it may be something that we look into this year.
So it looks like they are safe, however have never been tested to be certified as such.
A 25kg bag will give you approximately 9mm thick per square metre.
A plaster mould should be first sealed well with shellac and then a grease or oil applied. Halfords sell a LR Car Grease (a general purpose one) or a 20/50 engine oil can also be used. Apply these with a rag and by brush. It’s important to apply the release and leave for a short while before removing/wiping off any pooling or brush marks. You should only expect to get 5-6 casts from a plaster mould before it starts to wear.
There are 3 options, GRP (Fibreglass), Jesmonite and Plaster.
GRP is going to be the best in terms of quality of finish, durability and weight (lightest). Using a GRP gelcoat will give the best quality finish and the hardest surface, so less likely to wear or get damaged. You can also produce the thinnest, lightest mould with GRP.
Jesmonite would be the next best, although not quite to durable and heavier, it is obviously much nicer to use than GRP, as there are no toxic chemicals like GRP.
Plaster is obviously the softest least durable of the 3 however is probably the easiest to use and cheapest. Harder plasters will obviously be more durable.
Wax release (Macwax or Mirror Glaze) or Petroleum Jelly (Vaseline) will work best. However also most any release agent will work accept a silicone based release agent.
It will do to some extent, but will be a weak point. The old silicone must be completely clean and dust free, clean it with a solvent like Acetone or White Spirit, then simply apply your new silicone. It will adhere better when applied layer-on-layer (flat skin on flat skin) rather than end-to-end (edge to edge, if that makes sense!)
The relative density (or specific gravity) of 1kg of cured silicone is 1.21 (Silastic, Durasil and Smartsil) this yields 50 cubic inches or 819 cubic centimetres (cm). This can be calculated as follows;
100 square inches or 645 square centimetres at 1/2 inch or 12mm thick.
200 square inches or 1290 square centimetres at 1/4 inch or 6mm thick.
Clear Casting Resin
You can only cast Clear Casting Resin (which is a Polyester Resin) at a maximum of about 30mm at a time. This is because it does not have a filler content to it and thus overheats and cracks and crazes if cast thicker than 30mm. You can obviously build it up in layers, once one layer has cured. When building it up in layers you will not see any seam looking down through the cast, however you will see seams looking along the cast layers.
You should also note that you cannot cast Clear Casting Resin less than 5 mm thick! – It needs this thickness to generate enough heat to cure it properly.
You cannot casting Clear Casting Resin into Condensation Cure Silicones, but you can into Addition Cure Silicones. Most widely available silicones are Condensation Cure, Addition Cure Silicones tend to be harder, tougher and more expensive. We stock Silastic T-4 Addition Cure Silicone which work fine with Clear Casting Resin. Your silicone supplier should know whether your silicone is Condensation or Addition Cure.
Clear Casting Resin can be casting into almost any rigid mould; like glass, plaster, GRP, wood, plastic (but not polystyrene). However please ensure a suitable release agent is used, such as a wax like Mirror Glaze or Macwax.
Yes, basically 1ml = 1g for mostliquids. Strictly speaking most material recommend measuring by weight, but normally measuring liquid catalyst or curing agent by volume is accurate enough. However caution should be used if measuringbasesilicones orbaseresins by volume, as the SG (specific gravity or relative density) can vary a lot.
There are 2 parts to this, firstly the original must be sealed and secondly a release agent needs to be applied. For the very best results its essential both are done very well!
Sealing the plaster original. The key to this is having the plaster as dry as possible. Shellac is the most popular well used sealer, however really needs to be applied to dry plaster. You can also use a 2-pack polyester primer, however this also needs to be applied to dry plaster and also it is thicker than shellac so you will potentially lose any fine detail on the original.
Users tell us that our ‘Jenkins Transparent Shellac Sealer S055’ is one of best shellacs to use against damp plaster.
Applying releasing agents; You need to apply a good quality wax release agent like Mirror Glaze at least 4 – 5 times, buffing it up and leaving it 10+ minutes between coats (the more times you do this the better). And then it is recommended you apply a thin coat of PVA Blue Release, this isn’t essential, but is definitely recommended for an easier release and less likelihood of damaging your plaster original.
Resin will stick very well to shellac! So it’s essential a good covering layer of releasing agent is applied.
Our laths are made from Obeche or sometimes known as Wawa, which is an African timber. Although classed as a hard wood, it is relatively soft and flexible. It is pale straw in colour and is a light wood with a moderately fine and even texture. It is ideal for laths as it is basically knot-free. Primarily it is used to reinforce plaster cornice but is also used for lath and plaster ceilings, when riven laths are not required.
No we don’t sell Keenes Cement, however if you are wanting a material for Scagliola then we sell Herculite Hardcoat which is widely used for Scagliola.