Basically, there are a few "rules of thumb":
Simple, clean lines usually work best.
Things look smaller and farther apart in the casting - (make elements bolder/heavier and closer together than you might usually do on paper)
Sufficient depth is needed for the design to show up if viewed from any distance --narrow and deep molds can look good because they give good shadows, but they are difficult and labor-intensive to cast well.
|a very popular casting, but do you want to spend several hours putting pulp into crevices with a toothpick?|
There are several other things you might consider when choosing or working out a design for making a casting mold such as:
size, eventual use, difficulty (of cutting mold and of making a casting), legibility (this could mean "reading" or being able to see a design that's not lettering as well), as well as method of mold making (cutting vs.impressing or carving, etc.
Polymer clay molds can hold the tiniest details, but that doesn't mean they will be visible from a distance. If you are doing a card that will be held in the hand you can go small and detailed, but for wall art, you want to make sure it will not look like an empty frame hanging there. And very small things are not only hard to cut as a mold but are difficult to put pulp into when casting. However, you can successfully make impressed letters or designs very small -
When I teach beginning classes, I ask the students bring designs where the lettering (x-height) is not less than 1/2 inch high, preferably without hairlines or serifs. If you want it to be readable on a wall and easy to mold, you will need the clay layers about as thick as a matboard and your smallest design elements should be no smaller than that thickness.
Since the easiest method for transferring the design starts with a photocopy, make several in varying sizes so you can do some last-minutes adjusting, maybe even cutting and pasting elements from various sizes.
You will also have to consider the size of the oven. If you are being safe with a dedicated toaster oven, your mold does have to fit in it. Often an 8x10 tile fits well for baking on so this would be your maximum mold size including extra around the edges for making your lovely deckle, making the maximum design size more like 5x7".
|these are samples I bring for beginning classes to try out their first mold|
|The height of the taller sides are the width of my oven, baked on masonite pieces|
As mentioned above, you can go small if the item is something hand-held like a card or a book. You can make letter stamps from clay or do tiny writing by using the Clay Shaper to impress letters.You might be able to call attention to a wall piece with large letters or designs and have some smaller elements to discover as people come closer. Of course, the lighting where someone views it is going to make a big difference and you may or may not have control over that.
|This one is easy to cast but harder to see, I remade it in a smaller size to use as a card|
You will also want to consider how much time and effort you are willing to put into making castings. One of my most popular castings is also the hardest to cast (see second photo above), taking as long as three or four hours to make sure pulp is down in all the crevices and that colors are in the right places. This is not really a lot of fun and you are unlikely to sell them for enough money to make your time worthwhile (unless may you live in New York?). On the other hand, perhaps you only want to make one for yourself and many hours of work is not a problem.
Straight lines are good. Sometimes you can simplify just by cutting off serifs or other tiny details. Stencil designs are helpful to look at, they are usually bold and simple
As mentioned above, you will have trouble cutting if the width of the letter stroke (or design element) is less than the thickness of your clay layer. The simpler the design and the wider the area cut, the easier it will be to cast. Impressed letters, especially small ones, are fast and easy to cast.
|This student project is very crisp and clear and will be fairly easy to cast. Below it is a test casting of small impressed lettering across the bottom - a small trial mold was made and baked and cast.|
|paper cast Noel from polymer clay mold|
|the planning for NOEL, second and third try|
You can also make casting easier if you open up narrow areas by making it more of a wide "V" cut so the pulp can get in and out more easily. This can be done after the mold is baked and a test casting made, by scraping gently with your x-acto.
If no one can read the words or figure out the design it doesn't matter how well you cut the mold or cast it. It is the shadows that really show up, or the contrast between the highest white and the cast shadow. So some things can work with the letters going down and some won't. Here is an example of a design done two ways to see which would work best (lettering "up" or "down"):
|Its even more obvious in person, but the one on the right is so much more legible! In both cases the backing layer was textured, which puts the texture on the top part of the casting.|
And this is one that I never fixed or cast after the test because I was thinking that having "darkness" going down would make it darker but it was lighter because of less shadow. (If I were going to use this mold I would rub clay into the name at the bottom and bake a little and redo.)
|you can learn from my mistakes!|
Here's one where having the letters "going down" works just fine-
|This is a tiny one, used as an ornament or on cards.|
Multiple layers make it possible to have smaller and larger lettering or more complicated designs (just keep adding "backing") - the "windows" one above that I didn't like has three layers of depth plus "added on" words for the ones that go down in the casting.
If you want to impress instead of cut, you can make your own alphabet stamps of Premo, bake them, and use talcum to make sure you can get them out of the soft clay when pushed in-
|Paper Cast Book using alphabet stamps made from polymer clay|
these letters can still take some care to cast if you want them really clean and sharp.
Baked or unbaked clay can be carved with carving tools.
|A workshop participant in Salt Lake City combined cutting and linoleum and other tools|
Last but not least, all designs will benefit from having a borderline around them. Make it tighter in than you would think. Even if you don't use it to cut a raised or lowered border, you will need it to help make your deckle edge symmetrical - or as a guide to change where you put your deckle if the test casting shows its too near or far.
So, once you have the design chosen and its size worked out, you can decide how to prepare your clay (allow extra space around the edges, make layers thin if you are going to cut through more than one), transfer your design, and cut your mold. and bake.
Congratulations! next comes making your test casting.
Feel free to send design ideas for help and comment. (firstname.lastname@example.org)
and don't forget, I love to come and teach this in person- I'm better at explaining and problem-solving in real life than I am at writing.