SubscribeIt’s about time I got back to some real Pewter casting and the reporting of, so here’s my latest. I have recently been working on a token commission and actually paid attention to the detail of the results of the castings this time, so I thought I would write about it in an effort to further muddle it through in my mind as well as help the one or two of you out there.
When casting there are two main problems that cause artefacts on the cast item. The first is air pockets, and the second is pewter shrinkage due to the freezing process. Unfortunately in period soapstone casting, the solution to one of these problems is also what causes the other, and a balancing act needs to take place to minimise both effects.
Air pockets can be caused by two reasons that I have been able to work out, though I believe the second is another issue which can be fixed separately*. The first is a lack of air escaping the mould before the molten pewter freezes, either by air flow dynamics causing restrictions in air flow out of the mould as the molten pewter enters, or the pewter freezing before it gets a chance to fill all of the detail. This can be minimised by using a hot mould that allows the pewter to run into all detailed parts of the mould before it freezes, you may need to still play with the air flow dynamics on some moulds to ensure that the pressure of the liquid pewter entering the mould isn’t causing an air pocket in the mould to be trapped.
The secondary artefacts created when the molten pewter freezes and shrinks is a little harder to control, as the easiest way to control that is to have a cold mold and a quick pour of the molten pewter into the mould. The cold mold freezes a skin of pewter allowing the detail to remain. Freezing the pewter from the outside just a touch faster than it would with a hot mould this allow a harder skin to retain the detail whilst the pewter freezes and shrinks. Another way to stop the shrinking from effecting your finished casting is to have a large enough sprue connecting the pouring button with the cast product so that there is enough room for the shrinking pewter to pull down more molten pewter from the button, however this is not always easily possible, as this sprue needs to large enough to allow the sides to freeze before the centre.
Another method of minimising the effect of shrinking on the casting is to rapidly tap the mould on a wooden surface as you are pouring the pewter. This has the effect of increasing the pressure in the mould and effectively hammering the molten pewter into the details. It needs to be done as you are pouring, and you need to be very careful as you don’t want to move the mould parts, or splatter molten pewter.
These details leave me with the conclusion that in order to get a perfect casting each time from a gravity fed, non-pressurised soapstone mould casting there needs to be a very delicate balance between the temperature of the soapstone mould and the temperature of the molten pewter. If I were more serious about this process I would attempt to investigate this further with more period techniques for estimating the temperature of both mould and metal, however I believe that knowing that the balance exists and some means of minimising it’s effects are enough for me to continue. The art of getting a perfect casting from each pour is one that whilst very rewarding, is not necessary with the ease of turning out pewter castings it is often better to do the best you can, and recast the ones that don’t work. Of course, if it’s the techniques that are causing the artefacts in the first place, then these techniques do need to be addressed.
