CASSINI'S PLASTER
Cassini's is an extremely versatile water- and weather-resistant plaster-of-Paris, particularly suited for statuary, water-features, garden ornaments, bird-baths, plaques, mouldings, etc. and stonework restoration. As well as the normal fluid stage, Cassini's has a long paste phase - making it ideal for both casting and daubing work. Cassini's plaster is far more breakage-resistant than other plasters, making it the plaster-of-choice for many types of work both indoors and outdoors. Because it is so chip-resistant, it is also an ideal sculpting medium. Cassini's dries to a very pure white and is normally left natural ('as-cast') - but it can be can be dyed or painted, etc.
Mixing & Usage Guide
Unless similar pieces have been made previously, carry out a small trial piece to establish setting
characteristics. Cold conditions make mixing more difficult. Avoid working under 12ºC, so use warm water to ensure that the initial mix is kept above this temperature. For cast objects that are likely to be continuously immersed when completed (e.g. pond
features, fountains), use a ratio of 30:100 water-to-powder by weight (i.e. 0.3 litres of water per kilogram
of powder). For all normal outdoor work and all indoor projects, however, a weaker mix of about 35:100 is
quite sufficient. Always weigh the powder and use a calibrated jug, etc., for measuring the water. Most
plastics, latex, silicone rubber, paints, shellac, varnishes and insoluble waxes are all suitable mould
surfaces without any release agent. Note that the inherent moisture in alginate moulds can affect the surface. (If used, avoid liquid parting agents - these will adversely affect the
cast surface, although this won’t matter if the piece is to be sculpted anyway). Cassini’s is capable of
reproducing a higher level of detail than any other plaster, but this depends on proper mixing and good
technique. Avoid using your hands to mix the plaster as it is quite sticky.
Cassini's plaster has two stages of setting; the early flow stage is the same as other casting plasters, but after a brief transition stage, Cassini's can be daubed or pasted on, or used for bedding/jointing and restoration/repair work; lower down the page is the setting profile, with times plotted against the initial temperature of the mix. Small side-casts of 100g or so can be very useful in testing/monitoring setting progress, particularly in abnormal conditions. However, in normal circumstances, pouring of the plaster should be completed within approximately 8 - 9 minutes of commencement of mixing. Work with an assistant if mixing larger volumes of plaster - alternatively, build the casting up in manageable stages.
1. Measure out the correct amount of water; use tap water only. Use a clean container that will hold around 10x this volume of water to ensure plenty of space for mixing the paste.
2. Weigh out the required amount of plaster. Thoroughly loosen and break up any obvious clumps in the powder at this stage.
3. Mixing will take approximately 4 minutes; if mixing by hand, a more consistent and stronger plaster is developed if the plaster is mixed 30:100 to begin with (as if for a full-strength mix) or even less - adding more water up to the desired amount after a minute-or-so. Sprinkle the plaster into the water; the powder will not sink readily, so stir it in gently thoroughly. The mix will appear extremely dry initially but as stirring continues the paste will suddenly become much more fluid. Make sure there is no clumping in the corner of the bucket and that dry paste isn't sticking to the sides, etc. Add the remaining water now (if any) and mix it in thoroughly. Continue mixing until a completely uniform consistency is achieved. Tap the sides of the container and clear the surface of any bubbles before pouring. As with all plasters, do not add more water once mixing is completed.
4. If using Cassini's for normal casting, pour out the mixture gently and as near as possible to the mould to avoid splashing and air entrainment and, if possible, rolling the mould around and tapping it very firmly to ensure a thorough coating, etc. if using Cassini's for daubing or spreading on to a framework, existing plaster, etc. keep an eye on the mix to determine when it has made the transition to the daub stage, using the graph below only as a general guide.
5. The mix can be become quite warm and have a slightly lemony odour - depending on temperature, amount used, etc. Castings can normally be removed from the mould at about 1 hour. It is advisable to demould within 2 to 3 hours of casting to prevent a surface residue appearing on closed
surfaces.
The strengthening period for Cassini’s is longer than for other plasters, and new work will require
great care both during demoulding and handling - at least for the first few days.
The newly-set plaster will be off-white or beige and speckled or mottled, but Cassini's cures to a fresh, stone-white over the next few days; to ensure a more natural appearance on undecorated work, any unwanted gloss imparted by shiny mould surfaces can be removed by immersing the piece in water for 30 minutes immediately after de-moulding. Allow the piece to dry in a well-ventilated position or under-cover outside above 17ºC for 3 days, turning it regularly. The piece can be sculpted and carved much more easily during this period, particularly within the first few hours, but allow an extra day at the end for 'hardening-off' of cut surfaces. To prevent clogging of abrasive discs and sandpaper, etc., sanding and polishing is best left for 2 to 3 days. Avoid any painting, etc. during the curing period. Whether decorated or left as-cast, take the same precautions as with a natural material, i.e. protect from heavy frost, etc. (No sealer or primer is normally required for any type of decoration). Full mechanical strength and water-resistance is not achieved until approximately 5 days after casting, but finished pieces can be exposed to (intermittent) rainfall etc., after 3 days or so.
Because of its natural cementing properties, Cassini’s is a very effective bedding material for plaques,
plinths, tiling, etc., but it can also be used to repair existing stonework, including patching and reattaching
broken parts. Cut back, dry and vacuum the repair site beforehand. Drilled dowels may be necessary (non-
ferrous) when replacing damaged projections, etc. The plaster can be poured into a local mould but - for
small areas - the plaster can be applied with a knife
or small trowel after it has entered the daub-stage - alternatively, use a stronger mix (as low as 16:100 is possible) so that the
paste is stiiff enough to apply as soon as it has been mixed. A strong mix is also easier to work with
when using Cassini’s as a bedding, jointing or cementing material. The new plaster can be sculpted etc. approximately 3 hours after
setting, but avoid sanding or polishing for the first 2 to 3 days.
Like natural stone, Cassini's is water-resistant, but it is not completely impervious, so natural fibre reinforcement such as cotton, hemp and sisal should be avoided if the finished piece is likely to be immersed for long periods. Avoid permanent installation in wet, heavy or acid soils, etc.
Cassini's two stages of setting
The duration of the flow-stage of casting plasters depends on the type, water content, temperature, etc., and
is usually followed by a short transitional stage before rapidly stiffening. Cassini's has a flow-stage profile
similar to other plasters, but it also has a daub-stage during which it can applied with a knife or trowel,
etc. Both phases are sensitive to water content and the initial temperature of the mix - which can be
adjusted by chilling or warming the water or powder before commencing.
Chemical retarding: the setting profile of Cassini’s plaster can be extended by the addition of a small
amount of standard plaster retarding agents such as trisodium citrate. The retarding agent is best
dissolved in warm water first but avoid doses which are in excess of 2 to 3 grammes per litre of water used
in the plaster mix - greater amounts may cause darkening, weakening and even cracking of the finished
work.
Pigments and painting: Most cement dyes and polyester pigments, etc. can be used with Cassini’s,
however always carry out a small sample piece before undertaking any major work - if normal setting
appears unaffected there is very unlikely to be a compatibility problem. Emulsion paints can be mixed in at
the gauging stage. Once mixed and allowed to stand for a short while, gloss and oil paints can also be
added. Cassini’s can be overpainted, but follow the recommendations for surface preparation, primer and
undercoat that accompany the specific paint or aerosol. All paints will stay fresher with a clear varnish top-
coat.
Fillers: certain types of filler or additive should not be used with Cassini’s and these include wood, clay,
marble, cement and all other lime products, and any other plaster. Sand, granite chippings, glass, plastics
and other chemically inert substances do not normally present a problem but sample pieces should always
be carried out to ensure that the proportions used do not adversely affect strength and water resistance,
etc.
Although the presence of aggressive chemicals at the mixing/casting stage can cause problems,
unsatisfactory results with this plaster are almost invariably due to exceeding the recommended water
ratio, water being added after the initial mixing stage - or mixing/using the plaster below the recommended
temperature (see above).
Safety, Utensils, Storage.
1. Avoid inhaling powder dust; wear goggles and a mask and use gloves if you expect to have contact with the wet material or if you have allergic reactions to fine dusts, etc. To avoid build-up of odours, work in a well-ventilated area.
2. If you accidentally get any material in your eyes, wash out with plenty of clean water.
3. Wear sensible overalls or old clothing; store and keep the powder away from small children.
4. Use a bowl or plastic bucket for mixing, and use easily cleanable/disposable utensils. Avoid using metallic containers.
5. Low air or water temperatures will extend setting times, however avoid working at or near freezing conditions.
6. Remove accidental deposits or splash marks immediately.
7. Clean any non-disposable utensils and containers thoroughly in warm water immediately after use, but never down a drain or sink, etc. Alternatively, if using a smooth plastic bucket, etc, allow the excess plaster to set, then crack it off for disposal. At the latter stages of setting tools are more easily cleaned in warm water.
8. Store in a sealed bag, in a cool, dry situation, away from outside walls/solid lower floors, and use within 6 months.
9. Cassini's is considerably stronger than other plasters and once set is extremely difficult to remove from porous or rough surfaces such as fabrics, wood, etc.
10. Never attempt to dry any plaster in a microwave, otherwise the plaster may explode!
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