Even if you're fairly new to polymer clay, you've probably read that it's possible to cure things in stages-- to adhere raw clay to cured (baked) clay and put the piece back into the oven-- but you may not see why you'd ever want to do that. After all, it takes time to cure and wait for a piece to cool, and we're all about instant gratification, these days. For many projects, a single curing is all you need. However, there are times when curing in stages is very much worth the extra time and effort involved.
Here are a handful of reasons to consider curing polymer clay pieces in stages:
- Ease of Handling. Keeping clay free of fingerprints, nail marks, and other undesired textures can be a challenge-- even when wearing latex gloves (which can themselves leave textures and marks). Pre-curing a portion of your project-- a base, for example-- gives you a firm handle by which to hold and rotate the piece while you work on the remainder of it.
- Preventing Dust. As long as clay is "open"-- that is, uncured or raw-- it seems to act like a dust magnet. Even if you're careful about covering uncured pieces between claying sessions, it's common to find stray bits of dust, fluff, and so on that have somehow landed on your project. Removing them can be tedious. Whenever possible, you can prevent dust from ruining your work "so far" by partially curing it. Dust may still settle on cured pieces, but it's easy to wipe away when you're ready to recommence work (or play).
- Setting the Plasticizers. One of the reasons we condition polymer clay before sculpting it is to be sure that the plasticizers (the chemicals that make the clay malleable) are evenly distributed through the clay. You may have read (or noticed on your own) that even after clay has been conditioned, it can return to its original (right out of the wrapper) firmer state after it's been sitting around for a while. This is because the clay has cooled and the plasticizers have settled. It's a simple matter to recondition a ball or slab of clay-- just roll it through the pasta machine or scrunch and roll it between your hands. However, it's not such a simple matter to recondition clay that has been formed into a special shape and left to sit for weeks or months. You may cure such a piece and never have a problem, but some suggest that, if the plasticizers settle to the bottom of the clay, the strength of the clay may be reduced. Particularly if you plan to sell or give your work as gifts, you want to be certain that it is as strong and durable as possible. For this reason, I personally prefer to "set" the plasticizers in place by curing sooner rather than later. (When I've neglected to pre-cure, I've even scrapped partially constructed pieces-- putting the clay back to the recondition step-- rather than risk a weaker finished product.) Once cured, pieces can wait weeks, months, or even years for you to get around to the next step in your process.
- Ease of Cleaning/Working on a Solid Surface. For some techniques, I find that a solid surface (cured clay) is easier to work with than a soft one (uncured clay). For instance, when I make faux ceramics, I prefer pre-curing the textured "base" before applying the tinted liquid clay. This makes it easier for me to handle without fear of leaving fingerprints. It also means I can more easily wipe away the liquid clay, should I change my mind after applying it.
First, you'll have to decide, on a case by case basis, whether you trust the adhesion between the raw and cured clay to be sufficient. In many cases, it will be. However, when adding larger pieces-- or when the raw and cured clay share only a small surface connection-- you'll probably want to give the connection a little boost. You can use a dab of liquid clay (translucent, usually, but not necessarily) between the raw and cured clay. (Once cured, the liquid clay strengthens the bond between pieces.) You could also add a mechanical joint, such as a twisted piece of wire cured into the first piece and embedded into the second. Another option is curing the second (or third, or fourth) piece of clay separately and attaching it later (after everything has cooled to room temperature) with a cyanoacrylate glue (super glue).
Second, depending on your project and your plans, you can choose either partial curing (curing just long enough to firm up the clay and prevent it from taking fingerprints or dust) or complete curing (curing to the full length of time recommended by the manufacturers for the size of the piece). Partial curing is of course faster, and if you're going to finish the project and re-cure quickly, it's a perfectly good option. Also, there is less risk of color shifting than if you cure the "base" twice the required length of time. (You can reduce color shifting by using an aluminum foil "tent" over your pieces when you cure them and by mixing a little white or other opaque color of clay into colors that tend to shift.) On the other hand, complete curing makes the piece stronger-- less likely to break during any stresses it may encounter prior to final curing. If you plan to leave the piece for a very long time before returning to complete it, it may be wise to cure it completely. I've read horror stories about (accidentally) partially cured clay eventually crumbling (supposedly under the "attack" of unset plasticizers). It's up to you to decide which option is best for your particular set of circumstances.
- - - - - - -I think that covers the basics. If you've never tried curing in stages, maybe it's time!