Casting a hollow lead finial

[DJH]

I've just cast six small finials for a friends house and I've taken a few photos of the process so here goes......





This is the original pattern which I turned on the lathe and instead of having a pattern in two halves, which would have been easier, this is turned from a single piece of wood. For a short run of castings like this it's no problem to mould from the round but if there were a lot to do I would have glued two pieces of wood together making a sandwich of a piece of paper. This would then be mounted on the lathe making sure the centres line up with the paper joint. After the pattern is turned it's easily split on the paper joint and you now have two exact halves but that's for another time I reckon and it wouldn't make such an interesting topic.

 

[DJH]

Because the pattern is round with no flat surface it can't be moulded in the same manner as the lead name plate was by laying it down and ramming sand around it in a sand box. Any round object has a part or middle line which, in casting, is the parting line and to be moulded with sand the object has to be supported up to this line so that sand can be withdrawn from around the pattern.

I've used three boxes, a top, bottom and the third is used to bed the pattern in up to it's parting or joint line level with the top of the box. The two boxes on the left are the casting boxes and the one on the right is to hold the pattern bedded up to it's middle in sand.



The box in the middle has locating pins or dowels which line up with holes in both of the other boxes. This means that the all fit each other for the different procedures.

 

[DJH]

Now I've got the pattern bedded firmly in the waste sand box the next stage is to place the box with the dowel pins onto it... I've taken the pattern out to show how the impression in the sand isn't good enough to cast in.



After dusting the pattern with parting powder it's placed back in the box and the first half of the casting box fitted over it...



and moulding sand sieved onto the pattern as facing sand.

 

[DJH]

This picture shows how fine the sieved sand is compared with the unsieved sand at the back of the scoop.....



the sand is now rammed around the pattern taking care to do the perimeter sand before ramming on top of the pattern.....

 

[DJH]

the sand is strickled off. I've used a piece of oak flooring that happened to be near at the time but any straight edge will do...





The box is given a light tap around to help loosen the grip of the sand on the pattern...

 

[DJH]

and then lifted off revealing the impression of the pattern........



I've removed the pattern from the waste box on the right (which is now put aside ready for the next time) dusted both the sand in the casting box and the pattern and carefully placed it in the impression in the first casting box.......



and the second casting box is placed on top locating on the dowels of the box on the bottom.

 

[DJH]

and sand is sieved over the pattern and the box filled with sand and rammed.



pushed down around the pattern

 

[DJH]

and firmly rammed before being strickled off



I've now removed the top box.....



and you can see some sand from the second box has caught on the pattern but it can stay because it will locate back to where it came from when the other box is placed over it.

 

[DJH]

To allow lead to be poured into the mould I've cut away the sand in line with the holes in both the casting boxes





the two boxes are now ready to be fitted together....



the next stage will be the pouring the molten lead into the mould.

 

[MdB]

the next stage will be the poring the molten lead into the mould.

This is the bit that interests me. If it was chocolate, you would just pour in the melted chocolate, swish it around a bit and then pour it out.

But we aren't talking about melted chocolate, we are talking about molten lead in a rather heavy mould....

 

[DJH]

It's a similar process MdB but we don't 'swirl' it around in the mould as the molten lead freezes when it touches the sand in the mould almost instantly and any splashes in the mould would end up as laminations in the casting. When this happens the splashes can be peeled off the final casting which would then involve lengthy repairs.

The two boxes are now joined together with the wooden pattern removed,



This is a casting box I made for another job and I need to take off the two clamping plates to use on the finial casting boxes..



I've now have the boxes clamped together ready for casting. This is necessary as there is only about an inch or so of sand at it's thinnest point in the mould and the weight of the molten lead would cause it to burst open.

 

[DJH]

I've placed the box on the floor adjacent to the melting pot as it's all a bit heavy work for me nowadays.



Unfortunately Mrs DJH was unavailable to take the action shots ( shopping or lunch somewhere I suspect :lol: ) but I'm hoping to repeat the process again some time tomorrow, anyway, to continue, the casting box is filled to the top. The best time to tip the boxes and 'slush' the molten lead out is after two or three contraction rings appear at the top of the pour hole. I don't have any pictures of that yet but I'll post some tomorrow if I can persuade Mrs DJH to leave her Christmas preparations for a while and take some.

I've tipped the mould over and poured the surplus lead out of the mould and on to the metal which covers the floor of the workshop under the melting pot at this end of the workshop.



the boxes are now unclasped and opened showing the casting..

 

[DJH]

I've rolled up the slushed out surplus lead and back it goes into the pot.......





The casting removed from the sand

 

[DJH]

The used sand is knocked out of the boxes and the hot sand mixed in with the cooler sand and returned to the bin.



The two casting boxes and the waste mould box ready to repeat the process another five times



I've fettled the casting and now it needs a bit of filing here and there, a quick once over with a wire brush and that's it.



I think the camera makes them look as though they are bent a little.

 

[DJH]

I hope to post some pictures of 'chill' or die casting tomorrow if I can

 

[MdB]

So what sort of wall thickness do you end up with? Is the top part hollow at all or does that end up solid?

 

[DJH]

The thickness of the wall can be a bit hit and miss and is not easily controlled but I've found that leaving the lead in the mould for the time it takes to form three contraction rings around the pour hole is enough to give the casting integrity and also save on the amount of lead used. I would allow for reusing at least half to two thirds of the lead that's poured into the mould by using this method.

There are several variables of course, such as the temperature of the molten lead and the coldness of the sand. I have on occasion warmed the sand in the boxes, when the pattern has been removed, to compensate for the lead being too cool or to reduce the amount of moisture in the sand. This has to be done carefully as if the sand is overheated steam will come out and cause the lead to bubble when it's poured into the mould ruining the casting.

By the same token if the lead is too hot it will cause the moisture in the sand to steam and have the same effect. Having the lead too hot will scorch the sand by drying it out too quickly causing 'scabbing' or roughness on the surface of the finished work.

There are different ways of controlling the wall thickness one of which would be to make a core to fit into the sand boxes suspended by pins or chaplets. The lead is poured into the space between it and surrounding sand. This core would be made from a pattern the same shape as the original but smaller by the amount of wall thickness required for the final casting. This is a lengthy process so the slush method is used for speed, sacrificing a small amount of lead if the wall thickness happens to be a bit thicker in the long run. I hope that makes sense.

The end of the finial is solid as it is surrounded by cool sand and the lead freezes there quickly but as it widens near the ball section it is hollow. I suppose I should cut a casting open so that is visible.

 

[plasticpigeon]

That's interesting. Is the finial left like that or is it polished/machined afterwards??

 

[DJH]

It's now due for a bit of a rub over with a wire brush, not too much as any dust given off is dangerous if breathed in. I'll then dunk it in some rainwater and the silver grey patina will start to appear almost immediately. I'm heading to the workshop as I have to cast a small sundial as a present for some friends of ours who we are going to spend this evening with. If I tell him it's a bedside model and all he has to do is shine his torch at it to see what time it is, will he believe me?? :twisted:

Anyway I digress, I'll photograph the finials I cast last week to show how they have 'weathered' in.

 

[DJH]

After a last minute rush ( well I'd love to be able to rush ) I've taken some more photos of the finials and some slush casting using an aluminium chill, or die to give it it's proper name so here goes....

Here are the finials all displaying various stages of forced patination



This casting shows signs of scabbing where the sand is embedded in the metal because the molten lead was too hot



I've rubbed it with a coarse file to illustrate the difference between the scabbing and the lead beneath......