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Bronze, mould, core,plaster
When you cast bronze to make a casting with channels and cavities the major problems is to form the core and after casting remove it in a simple way. One method I have used is to make a silicon mold of the core and fill it with foamed plaster of Paris.
These foamed materials is made by taking water and add as much plaster as possibly while it is still possibly to stir it with an electric mixer. When it is thoroughly mixed you add a detergent which will make it easy flowing again at this point you can add more plaster to give it a suitably viscosity. Whip it intensely with the blender until the volume had increased 30 – 50 %. Fill a syringe or a used glue gun with the foam and inject it into the mold, it does not harden as quick as expected because the detergents interfere with the crystallization.
The reason why I first mix the plaster without detergent is that by this way I get small even sized air bubbles. Since the material is very fragile, it is best to reinforce smaller sections with pipe-cleaners which also work as vents. After a day or two the material can be taken out of the mold and dried. The foamed plaster is now heated to 500 oC but not more. Make sure that you have good ventilation because it smells. If it looks too fragile it is possible to impregnate with a 20 % solution of linen oil in petrol and harden it at 200 oC Make sure that all petrol is gone before you put it in your own. That is most convenient done by heating it with a heat gun first.
The core can now be put into a silicon rubber mold and be fixed by pins. Wax is poured into the mold leaving prepared vents open. Since the core cannot withstand more than 500 deg. it is not possible to use the high tech. shell casting technique, instead the clay, mineral fiber and paper method is recommended. The core can also be used in sand casting but remember the enormous lifting force from the molten metal.
This is a picture of what is needed to make a foamed plaster core. For bigger cores the glue gun is recommended. In the picture is also a core to make the shaft hole in an axe in sand casting. It is recommended that you drill a hole and insert a long rod to decrease the lift from the metal.
This is a pattern of the rim of a wheel for sand casting. The holes for the spokes are made of solid plaster when the pattern is set. This is changed for foamed plaster before casting.
This is a core made of foamed plaster reinforced with pipe cleaner. To the right is the finished umbrella handle which is part of my fountain.
I tried several times to cast this honeycomb with the high tech shell casting method but failed every time. Then I made a print of the side with hexagons with foamed plaster and used that inserted in a sand casting mold. This worked perfectly with both bronze and aluminum.
I found that artichokes would be very decorative so I made a silicon rubber mold of it. This I filled with about 100 g molten wax and shake it until it solidified. The hollow space was filled with foamed plaster using a glue gun. Two strands of pipe cleaner were put down into the plaster to make a vent through the narrow shaft. The wax was now imbedded in the clay – mineral fiber – paper – wallpaper glue mixture to a dry thickness of about 5 mm. Small nails was hammered into the core to fix its position. A little clay mixture was put over the nails. Some small holes were drilled below the shaft to give better exit for wax and gases. When dry the wax is melted out with a gas burner but not overheated. The mold is heated in a ceramic kiln to 500 oC to make sure that all combustible material is burned. To improve the surface you can impregnate with linen oil as mentioned before. The mold is now put in supportive sand and filled with bronze.