Hi Sylvia,



A couple of years ago I wrote up a few of the techniques we use in Cardiff for casting (please see below). There are some materials suggested in the text, but like you say, there are lots of materials out there and the actual choice very much depends on many factors: the size and shape of your mould, the level of detail you require, and what the casts are going to be used for. You need to consider desired longevity of the product, viscosity of the casting medium and the speed of the curing process. Also bear in mind that polyester resins are exothermic and if you have a large mould the heat produced can be considerable! 



Regardless of what you decide on doing in the end I would recommend practising your casting first before using the borrowed moulds. Silicone moulds don't last forever, and some resins can be quite aggressive on silicone, so the more the moulds are being used the more they will deteriorate over time. 



Best wishes

Christian





Christian Baars

Senior Preventive Conservator

National Museum Cardiff

Cathays Park

Cardiff CF10 4NP

+44 (0)29 2057 3302

[log in to unmask] 

Twitter @NMWPrevCons







Casting – Epoxy Resin 



Summary: Small to medium-sized fossils can be cast in epoxy resin. This has minimum shrinkage and maintains its size and shape better than other synthetic resins, even though it has the disadvantage of being quite expensive. 

Location: Palaeontology Preparation Laboratory, Geology Laboratory.

Equipment: Fossil mould, epoxy (Araldite DBF resin with Aradur Hardener; Robner Resins Ltd, Swindon SN3 4TE, 01793 823741), plastic cup, wooden stick, brush. 

Technique: 

-	Prepare the fossil mould (usually silicone mould; refer to Moulding – Making a Silicone Mould) and ensure it is free of dust.

-	Mix resin as 1 part hardener and 5 parts resin or per manufacturer instructions.

-	Apply initially to mould by brushing on the resin to avoid the formation of bubbles on the mould surface, then pour in the resin to fill the mould and tap the mould to release any bubbles. 

-	Leave to go off over night.

N.B.: Work in a fume cupboard as some of the components are hazardous to health. 

N.B.: Pigments may be added to the otherwise clear resin. Some pigments may also be painted directly onto the mould before pouring in the resin, giving a more natural surface appearance. 

N.B.: The curing process of epoxy resins is exothermic, and the amount used for large casts can generate a substantial amount of heat. In this case, consider making a laminated casts, where several successive thin coats of resin are applied to the mould. 





Casting – Jesmonite 



Summary: Medium-sized to even very large fossils can be cast in Jesmonite acrylic resin. Jesmonite has the advantage over plaster that it is stronger and less fragile. 

Location: Palaeontology Preparation Laboratory, Conservation Laboratory.

Equipment: Fossil mould, Jesmonite (AC300, liquid and power parts; Jesmonite Ltd, Challenge Court, Bishops Castle, Shropshire SY9 50W, 01588 630302), plaster mixing paddle and electrical drill, bucket, brush. 

Technique: 

-	Prepare the fossil mould (usually silicone mould) and ensure it is free of dust.

-	Mix Jesmonite as 1 part liquid to 2.5 parts powder; mix well to avoid lumps forming. If mixing larger quantities use a bucket and an electrical drill with a plaster mixing paddle attached. 

-	Apply the first layer of Jesmonite to the mould using a brush to avoid bubbles; pour the remainder of the Jesmonite into the mould. With large specimens work reasonably quickly as Jesmonite starts to go off within ca. 15 minutes. 

N.B.: Large specimens may be strengthened by incorporating sheets of glass fibre with alternating layers of Jesmonite. 

N.B.: Jesmonite AC300 has a relatively low resin content and is only suitable for internal applications. Casts for external applications can be prepared using Jesmonite AC100 with a suitable sealer or external paint for long-term exposure. 





Casting – Latex Rubber 



Summary: Small- to medium-sized fossils may be cast using latex rubber. 

Location: Palaeontology Preparation Laboratory.

Equipment: Small dish with water, fine brushes, fine needle, liquid latex rubber (e.g., Trylon Ltd, Bury Close, Higham Ferrers, Northants NN10 8HQ, telephone 01933 411724), latex colourant, modelling clay (plasticene [e.g., Lewis Newplast; Newclay Products Ltd., Heathfield, Newton Abbot, Devon TQ12 6RY, UK] or Farcolina modelling clay [Chemia, Post Boks 72, 3850 Ølstykke, Denmark, telephone ++45 47175522]), cotton gauze. 

Technique: 

-	Cleaned and consolidated specimens are prepared for latex casting by forming a containing wall around the fossil. Rolls of modelling clay ca. 5 mm in diameter are shaped in approximately circular fashion around the fossil. Depending on the size of the fossil, several layers of rolls are formed on top of each other to the required thickness of the latex casts (ca. 5 mm for fossils with a diameter of less than 2 mm up to c. 20 mm for fossils larger than 30 mm). 

-	Water is brushed carefully onto the fossil, particularly into areas such as spines or fine gaps, to subsequently aid the latex to penetrate these fine details. With non-porous or previously consolidated rocks, the addition a small amount of some surfactant (e.g., washing up liquid; however, the use of too much surfactant will create bubbles) assists in the formation of an even thin film of water. 

-	A thin film of latex liquid rubber is then applied to the moistened fossil, taking care to avoid the formation of air bubbles; if necessary, bubbles are removed using a fine-tipped needle. 

-	Once the first layer is dry subsequent layers of latex are added, allowing each layer to dry completely. 

-	Finally, a strip of cotton gauze is applied using a fresh film of latex. The modelling clay is then removed, the cast separated from the fossil and checked under the binocular microscope for the absence of bubbles. 

N.B.: For research specimens, a few drops of red latex colourant are added to the liquid latex prior to application to enhance the contrast of any photographs taken subsequently. 

N.B.: Latex is not very stable long-term; it tends to shrink and curl up at the edges. One way of counteracting this is to strengthen the cast by incorporating cotton gauze (see ‘Technique’). However, it should be noted that a latex cast is likely to be marginally smaller than the original fossil and if a faithful replica is required for instance for research or collection purposes a different material should be considered. 





Casting – Plaster 



Summary: Plaster is a material that allows quick, easy and cheap casting of fossils. 

Location: Palaeontology Preparation Laboratory.

Equipment: Fossil mould, dental plaster, water, suitable plastic container, spoon or other stirring device.  

Technique: 

-	Prepare the fossil mould (see: Moulding – Making a Silicone Mould) and ensure it is free of dust.

-	Mix plaster with water to give a consistency slightly thicker than pancake mixture. Slowly pour plaster powder into the centre of a bowl or cup with water until the plaster forms a little island in the water; mix thoroughly. 

-	Pour the plaster into the mould and tap mould to release any bubbles. With larger specimens work reasonably quickly as plaster starts to go off within ca. 10 minutes. 













-----Original Message-----

From: The Geological Curator's Group mailing list [mailto:[log in to unmask]] On Behalf Of GEO-CURATORS automatic digest system

Sent: 05 November 2014 00:02

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Subject: GEO-CURATORS Digest - 3 Nov 2014 to 4 Nov 2014 (#2014-115)



There are 4 messages totaling 1234 lines in this issue.



Topics of the day:



  1. Casting fossil replices (2)

  2. FW: Using natural history collections in museums- some tools to help

  3. FW: Review and future planning for the Culture Grid



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Date:    Tue, 4 Nov 2014 10:01:54 +0000

From:    Sylvia Humphrey <[log in to unmask]>

Subject: Casting fossil replices



Dear All







We are casting some fossil replicas from borrowed silicon rubber moulds.  We would appreciate advice re suitable resins (a web search has revealed that they are many and varied), especially from colleagues who have recent experience of this type of work.







Many thanks



Sylvia







Sylvia Humphrey



Assistant Keeper of Geology



The Great North Museum: Hancock



Barras Bridge



Newcastle upon Tyne



NE2 4PT.



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