Maker Photo Diary (continued)

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Prepping the plywood master + framing up

Fine finishing and applying a third coat of polyurethane varnish to the laminated plywood master. Firmly fixing the master to the melamine base board with wood screws and attaching 25mm x 20mm battens to frame up the casting area, allowing a 20mm wall thickness around each edge of the master.

Mixing the silicone

For the  mould RTV-3481 a market leading condensation cure silicone was employed. RTV-3481 offers outstanding tear strength and high elasticity for easy de-moulding of complex replica parts (PS Composites). The 2 part silicone base and catalyst were mixed together at a ratio of 10 parts silicone to 1 part catalyst, calculating the amount required by volume and accurately weighing the quantities. The combined liquids where then stirred thoroughly until an even grey coloured consistency was achieved.

Pouring the silicone

The mixed silicone was then poured over the plywood master in an even action. To avoid pockets of trapped air and ensure the silicone filled all small recesses, the mix was poured at height approximately 600mm above the master former. Once filled level to the top of the battened frame, the mould was agitated by raising and tapping vigorously on to the bench bringing any pockets of air to the surface.

Removing the silicone cast

The silicone cast was left overnight to allow curing. The next morning the cast was carefully removed from the plywood master, using extreme care not to tear any of the fine detail, to finally reveal the negative former. Small deposits of overspill on the underside of the plywood were caused by very slight warping of the melamine base board, which made it difficult to separate the mould from the master. By removing the plywood master from the base board it was simply a case of slicing away the residue silicone to allow the mould to be fully detached. 

Fine finishing the silicone mould

Unwanted residue of silicone was removed from the mould with a scalpel. Fixing point positions for the chair frame were then transferred from the plywood master to the silicone mould and finally, a coat of silicone lubricant was applied to the surface of the mould to reduce the possibility of material sticking during casting. 

Preparing for the first casting

To create diffused colour within controlled sections of the cast, a laser cut card template of the pattern was prepared with areas masked out. Brown and yellow powdered cement dyes were then sprinkled over the exposed areas so they could settle directly on the silicone mould below.

Mixing the concrete

The concrete mix combined one part Portland cement with two & half parts building sand and one & half parts 4mm grano. Before adding water and plasticiser, 35ml of powdered black dye was added to the dry mix and folded in. The dye also acts as a plasticiser so, the amount of liquid plasticiser was reduced by half the recommended volume and folded into the mortar mix with one & half parts water.

Casting

The concrete mortar was applied to the silicone mould using a trowel, firstly to the areas where the dusting of coloured dyes lay, taking care not to disturb unnecessarily. 4mm steel rods were strategically placed in the mould at the half depth stage to reinforce the mix before filling level to the top of the mould. The concrete mortar was tampered and screed finished using a straight piece of timber, as shown in the next set of images and video clip below. 

5mm diameter rawl plugs were cut down to 12mm lengths and pushed flush into the surface of the concrete using the pre-marked guides. Finally, the front edge of the mould was elevated to create the same profile curve as on the tubular steel frame. This was achieved by using a card template and packing the underside of the silicone with wooden profiles then clamping a length of 15mm Copper tube to the face side of the mould accentuating the curve and holding the position whilst the concrete was left to cure.

Exposing the cast

The mix was left for 48 hours to set and begin curing before removing from the silicone mould. The silicone mould came away from the concrete without any resistance, and once fully exposed, the concrete was left for a further 24 hours to continue curing. The quality of cast was very good with just a minor amount of detail not quite as defined in one area but, not enough to detract from the overall appearance of the casting.

Second casting (alternative approach)

After cleaning the silicone mould a second mix was created using the same recipe but, this time increasing the black dye and liquid plasticiser to the manufacturer's recommended amount. Instead of applying a dusting of dry cement dyes directly to the mould as, this time a small amount of the mortar mix was separated before adding the the black dye and water. 

The separated mortar was split into two quantities with yellow dye added to one and red dye to the other. Once each quantity of concrete mortar had been mixed the coloured quantities where trowelled into the leaf areas of the mould before adding the black mortar to the entire surface of the mould.

Once again after leaving to set and cure the silicone mould was removed with ease however, this time the result was not successful. Despite the high quality and finish of the cast, a hairline crack was evident across the centre and when placed on the bench it immediately broke into two pieces! 

This could have been the result of removing from the mould too soon after casting or, possibly by increasing the amount of both black dye and plasticiser which compromised the consistency of the mix. Purely from an aesthetic value this second cast did not capture the autumnal quality aimed for as achieved with the first version.

Third times a charm

After the unsuccessful previous attempt, a third cast was made using the original recipe and approach but reducing the dusting of coloured dye and increasing the volume of liquid plasticiser very slightly. This provided a cast that was both strong in form whilst capturing the autumnal appearance of fallen leaves aimed for. 

During a period of curing the concrete was sealed and finished with three coats of SBR (styrene-butadiene rubber)  waterproof bonding agent and sealer, diluted three parts water to one part sealer. This provided a resistant satin finish to the surface whilst improving durability of the cast.

Finally, the seat was lightly buffed with a fine coating of Renaissance wax applied on a lint free cloth before being fixed to the chair frame with 35mm long twin fast screws.