8.    Cast decorations in plaster, gypsum or cement on facades

Quite a lot of the decorations on plaster facades are cast in Portland Cement mortar, gypsum or other specially mixed materials and thereafter mounted to the plaster surface. 
Portland cement is created among other things to suite this purpose, but it is hard to believe, that a relatively weak material as gypsum can last for long time on rain, frost and salt exposed exterior facades. But experience shows, that cast gypsum decorations have a surprisingly good durability and can under certain circumstances last for several hundred years.
The gypsum decorations are in spite of this mainly reserved to quite flat reliefs or friezes, or they are placed well sheltered from water under a large main cornice or roof eaves. It is furthermore very important that the cast gypsum decorations on facades are surface treated and protected by a layer of linseed oil paint.
A very special material, used in Britain as early as the late 1700s and still used today is composition or comp, a very early "plastic" material, consisting of linseed oil, animal glue, natural resin and clean whiting as the filler. Heat and damp was used to  speed up the curing. 
The main material for cast decorations is however Portland Cement, which appeared as a very popular and used product for this purpose at about 1860. It is quite often difficult to see, whether a facade decoration is hand made on the spot or cast and mounted. Only more close investigations will disclose or verify this. The treatment in case of damages and deteriorations are quite different.

The cast decorations were often cheaper than the hand made, as they can be mass-produced and still relatively individual, because of the huge amounts of products and enormous sales potential. Complete catalogues with drawings, sizes, variations and prices were sent to the crafts firms, to the architects and the building owners from the big factories in each country. Columns, half columns, balconies, balustrades, consoles, casings, friezes, festoons, medallions,  etc. etc.

There are three methods for casting in Portland cement:

1  Pressed semi-dry casting method in a rigid reverse piecemould
2 Wet casting method in a rigid reverse piecemould
3 Wet casting method in a flexible mould

When recasting old facade ornaments on historic houses to day, primarily the flexible moulds are used, so only this method is described here.
The advantage with the semi-dry technique is, that it is far and away the fastest, as a cast takes 20 to 30 minutes. A wet cast demands a day in the mould to harden.
It is recommended to use ordinary Portland cement (OPC) for this purpose, in suitable mixes with selected coloured sand 1:3 - without the many modern adherents and reinforcements as fibreglass (GRC), PVC and others 

Wet casting with gypsum or Portland Cement in flexible reverse moulds:

1 Production of a full scale model in a appropriate material: Gypsum, wood, cement. 
2 Making of a negative casting mould in cement, gypsum or, to day, silicone rubber. The mostly used type is ìopenî in the back, only with decorations on the three sides - plus possibly top and bottom.
3 The casting mass, Portland Cement, gypsum, more rarely hydraulic lime mortar, is poured into the casting mould. Irons or other systems for mounting are fastened in the wet and soft material
4 After the hardening the cast element is removed carefully from the casting mould and possible defects are repaired or retouched.
5 Finally the cast element is sold, transported to the building site and mounted at the facade.

Mounting systems

Drawing no. 23 shows the various mounting systems, anchors, glue, even simple wet gypsum mass. Other methods can be used and must be explored from case to case. 
The iron anchors cause the heaviest problems, as they will start to corrode, when possible cracks or gaps allow water to expose the iron surface. The inevitable result is corrosion which again soon will cause bursting and breaking masonry because of the expansion of the iron.

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