One of the first decisions you make when approaching a new slipcasting project is how many parts your mold needs to be. Get it right and the piece releases cleanly every time. Get it wrong and you either can’t demold the piece at all, or you end up with seam lines in the wrong places. Here’s how to think through it.
The Core Rule: The Pen Test
The simplest way to determine how many mold parts you need is the pen test. Hold a pen vertically and run it down the side of your object from top to bottom. If the tip of the pen disappears from view at any point — meaning it passes behind an undercut or overhang — that area cannot be released from a one-part mold. You’ll need a two or multi-part mold to accommodate it.
Think of it this way: plaster molds are rigid. The cast piece has to be able to pull straight out of the mold without any part of the mold blocking its path. Any shape that “grabs” the piece as it’s being removed requires the mold to come apart to release it.
One-Part Molds
A one-part mold is the simplest option — a single block of plaster with a cavity that the piece releases from by lifting straight out (or in the case of slipcasting, inverting to drain and then separating).
Best for:
- Simple open forms — bowls, plates, dishes, shallow trays
- Pieces with no undercuts or overhangs
- Forms that taper outward from the base upward (or the reverse for press molds)
- Tiles and flat slabs
Advantages:
- Fastest to make — one pour of plaster
- No seam lines on the piece
- Easiest to use — no assembly or alignment required
- Less plaster, less storage space
If you can use a one-part mold, use a one-part mold. Always start here and only add parts when the form requires it.
Two-Part Molds
A two-part mold splits the object along a parting line — the seam where the two halves of the mold meet. When the cast has formed, you separate the two halves and the piece releases from both sides simultaneously.
Best for:
- Closed forms like mugs, cylinders, bottles, and vases
- Pieces with undercuts that can be divided along a single parting line
- Most standard functional pottery forms
- Pieces where the seam can be hidden or easily cleaned up
Key considerations:
- Parting line placement — where you split the mold determines where the seam appears on the piece. Plan this carefully. For a mug, the parting line typically runs vertically down the sides, hidden by the handle attachment points.
- Natches — you need registration keys (natches) between the two halves so they align perfectly every time. These are typically small hemispherical depressions and bumps that interlock.
- Seam cleanup — every two-part mold leaves a faint seam line. Plan for light cleanup with a sponge or fettle knife at the leather-hard stage.
Multi-Part Molds (Three or More Parts)
Some forms are complex enough that two parts aren’t sufficient — the piece has undercuts or features that require the mold to come apart in three or more directions to release cleanly.
Best for:
- Handles, spouts, and attachments cast separately
- Complex sculptural forms with multiple undercuts
- Pieces with features that protrude in different directions — like a teapot spout and handle
- Production pottery where handles are cast separately and attached after
Key considerations:
- Complexity compounds — every additional mold part adds time to both the mold-making process and each casting session. Only add parts when genuinely necessary.
- Separate part molds for attachments — for handles and spouts, it’s often simpler to cast them in their own small two-part molds and attach them to the main body at leather-hard stage rather than trying to incorporate them into the main mold.
- Mold storage — multi-part molds take significantly more storage space. Factor this into your decision, especially in a small studio.
Decision Flowchart
- Run the pen test — does the pen tip disappear at any point?
- No One-part mold. You’re done deciding.
- Yes Can a single parting line eliminate all the undercuts?
- Yes Two-part mold. Plan your parting line carefully.
- No Do the remaining undercuts belong to attachments (handles, spouts)?
- Yes Cast attachments as separate molds. Keep the main body as a two-part mold.
- No Multi-part mold. Map out every parting line before you pour any plaster.
Practical Tips
- Start simple — if you’re new to mold making, work with one-part molds until you’re comfortable with plaster mixing, sealing, and demolding before moving to two-part molds.
- Prototype in clay first — make a rough clay model of your intended form and do the pen test before committing to plaster. It’s much faster to adjust clay than to remake a mold.
- Draft angles help — designing your forms with slight outward taper (draft angle) from the deepest point makes release much easier regardless of mold type.
- When in doubt, split it — a two-part mold that releases cleanly is always better than a one-part mold that traps your piece.
For step-by-step guides on making each type, see Making One Piece Pottery Molds and Making 2 or Multipart Molds. For the plaster mixing side, see our Plaster Calculator and Mixing Guide.
