Hydraulic limes have a chemical set similar to cements and can be feebly hydraulic, moderately hydraulic or eminently hydraulic (which is also called natural cement), the material is best mixed in a forced action pan mixer such as a ‘ SoRoTo‘ which we use and which gives good reliable consistency and disperses the hair (if added) evenly and generally applied to masonry backgrounds or timber lath backgrounds but can also be used on metal lathing usually applied in 3 coats.
First coat scratch coat (on timber lath or metal lath called the pricking up coat) 10mm- 12mm thick lime and sand plaster (on lath haired lime and sand).
Second coat or floating coat 12mm – 15mm thick in lime & sand plaster (or haired lime and sand on lath) If there are any severe irregularities they can be ’dubbed out‘ between the first and second coats.
Finish coat lime putty – fine washed sand – 1:1 usually 3-4mm thick applied in a min of two coats. This finish coat is not ‘polished’ with the trowel as gypsum plasters.
Timber laths for this work can be sawn or riven depending on the age of the building being restored.
Riven laths are expensive and only available in short lengths – 1.2m and can be of Oak, Chestnut or Scotch pine and these can vary considerably in width, thickness and shape but are usually around 25mm -30mm wide and 4mm – 8mm thick.
Sawn laths are usually supplied in 2.4m -3.00m lengths and can be of Oak, Chestnut, Pine, Deal or Larch and are of a much more uniform shape and thickness usually 25mm wide x 6mm thick. The laths are fixed to timber joists using stainless steel or galvanised ring shank lath nails 30mm – 40mm long and the joints staggered every 8-10 laths.
GRG - Glass reinforced Gypsum is a similar material to Fibrous plaster with the difference that the reinforcing materials are continuous filament glass mat and usually metal instead of timber, this improves the strength and fire resistance of the material, the plaster used is a harder & denser type called ‘Keramicast’.
GRG units can be made very light and strong, units of up to 8m2 can be produced, the material is very suitable to the manufacture of large curved units such as domes, vaulted ceilings, columns and large curved shapes in general and can be jointed to a seamless finish, GRG is usually fixed to a metal framework and can be screwed, bolted or wire and wadded, but can be fixed the same as Fibrous plaster to a variety of substrates.
pmGRG is polymer modified GRG which can make units lighter and stonger and add a degree of water resistance, using this material cast thickness can be significantly reduced, panels made lighter and stronger making transportation and fixing easier.
Jesmonite AC 100 is a propriety material for the manufacture of pmGRG which has the advantage of being a ready to use product of known consistency & quality which we use when called on to manufacture pmGRG, this material offers a variety of finishes such as stone, metal, smooth or patterned etc, the pmGRG units are made modular with designed / expressed joints as they cannot be jointed.
Fibrous plaster is a traditional material which is light in weight compared to the solid run in situ finishes that preceded fibrous plaster and is used mainly for the manufacture of decorative elements such as cornices, ceiling centres, panel mouldings & corbels etc, and has big advantages over the ‘run insitu’ methods as it can be made off site, stored until required and the installation is quite rapid and relatively clean & dry.
The material is made from fine casting plaster reinforced with hessian and timber laths, it can be cast into complex shapes the limits being the characteristics of the type of mould used which can be of plaster, GRP ( Glass reinforced polymer ) for long production runs, timber or silicone for intricate ornate work.
Once cast & dried the units can be fixed with adhesive, screws or a combination of the two or they can be wired and wadded to timber or metal framework, it can be fixed to a variety of substrates. The material is easy to cut, drill, screw and make good with casting plaster to a seamless finish.
GRC Similar production methods to GRG except that the binder is cement instead of gypsum, the products even look similar. GRC products have the advantage of being waterproof and is suitable for external precast finishes such as stone finishes, external mouldings such as cornices, window surrounds, cills, pediments, columns, balusters, copings and other architectural features. The GRC units are made modular with designed/expressed joints as the material cannot be jointed.
pmGRC Polymer modified GRC - this product is used for the manufacture of precast external architectural features - usually these are of a modular design with expressed joints. They are light in weight waterproof and can be made to simulate stone, metal & even timber.
Jesmonite AC 750 is a propriety material which has the advantage of being a ready to use tried and tested product which we use for this type of work, this material also offers a variety of finishes but is particularly suited to stone finishes.