Multiple firings and joining silver clay pieces

[silverleaf]

Hi,
Just come across this forum, and it's excellent. So many tips!!!

I've searched though and can't find any specifics on multiple firings. We're making a complex piece and to avoid the risk of cracking the parts before assembling, are wondering whether we can fire the individual pieces and then join with paste to assemble them into one final complete item? Will it be less strong than if we take the route of assembling with paste while clay, and then firing the whole thing? Just wanting to reduce the risk of cracking parts of the design, so if it makes no odds we'd fire the bits separately and then join them together. Anyone had experience of this?

Fyi, we're using ACS 650 and will be firing with a torch....

[Metalman]

Well first off - your project sounds great but potentially large. I want you to make sure you can manage the torch work on a larger project. The rule of thumb is that generally you should only fire 25 grams or less. Any bigger than that and the recommendation is that the piece be fired in a kiln.
So with that proviso - I have done a lot of multiple firings with and without metal inclusions. Usually I build the basic structure - possibly the complete piece. Sometimes you will want to do a basic structure - fire it - then do additional building and ornamentation on that structure; finally a finish firing. When you build on metal inclusions or on fired clay - the new clay you add on will shrink in the next firing - So you want to have a good joining surface and pretty much plan firing again to fix any cracking or shifting that may have happened.
I can usually do everything I want in 3 firings. I use a combination of ACS650 and the Overlay paste. The overlay paste works extremely well at bonding fired clay and unfired clay. I use 2 thin layers on fired materials to provide a footing for my additional builds the coat over lightly on top. Then I fire and use Overlay paste with small amounts of clay to repair and fill any cracks or blemishes [usually occurring from shrinkage]  It has been my sense of the materials that multiple firings make you Metal Clay pieces stronger and more durable but I don't have any science to prove that.
I ahev built a lot of work this way - it may take a
couple of test pieces for you to work out your methods - so do a couple of tests before you work on that Master Piece you are planning on

[silverleaf]

Thanks, that's great to know. It sounds like our safest option will be to fore the pieces we already have and then jointhem. The overall weight of the piece will be 15g, maybe 20 so we're not worried about the size.

For the overlay paste is there any special technique? I've seen people mention oil paste but is normal ACS650+water good enough?

Thanks again for the advice.

[Metalman]

Overlay Paste is formulated primarily to do ornamentation on glass and glazed or vitrified ceramic [does not work so well on lowfire, terracotta or soft ceramic] Works great on porcelain, high fire stoneware or glazed items.
This special formulation seems to enhance the bite and connection to metal inclusions and already fired clay - so I use it as the connector/footing for my additional builds - I have had very good success with this but you need to follow a few basic methodologies.

• For inclusions of other metals - make sure there is a physical connection as well as the clay; this might be as simple as coarse texturing the metal so there is tooth for the ACS to bind into
• For fired clay, do not bush , sand , burnish , or finish the piece until you have done all of the firing you intend to do; the surface as it comes out of firing works well to build on but any of the finish techniques will smooth it out and it will lose good connecting tooth
• Firing hotter is hazardous so make sure you balance your torch firing to a good  - peachy orange and hold it there; firing longer can enhance and strengthen your piece [but not hotter]

Testing and experimenting is important to developing these methods and making sure they work for you
Keep asking as you need