Hello! Welcome to the second module of our course, "From Model to Object: Medal Production and Finishing."
In our last lesson, we concluded our design module by focusing on how to translate a 2D design into a 3D low-relief model. We explored conceptual frameworks like "The Flattening" and "The Deepening," and practical digital techniques in Blender. A recurring theme was the need to respect technical constraints, such as draft angles, which are dictated by the manufacturing process.
Today, we'll dive deep into those constraints. Our goal is to distinguish between the production requirements for cast versus struck medals, focusing on relief depth, draft angles, and undercuts. Understanding these differences is absolutely critical, as the choice between casting and striking sets the rules for your entire design and modeling process.
The Two Paths: Casting vs. Striking
Medals are primarily made in one of two ways: by casting or by striking. Given your extensive background in sculpture and metal casting, the first process will be conceptually familiar. The second, striking, is more of an industrial process related to coin minting. Let's look at both.
First, let's watch a brief overview of a sand casting process. While you're familiar with other mold types like silicone and plaster, sand casting is a classic method for creating medals.
Making a Medal Using the Sand Casting Process
This video from The Frick Collection, 'Making a Medal Using the Sand Casting Process,' shows the entire workflow from wax model to finished bronze medal.
Please watch from the beginning to the end (00:06 - 04:02). Note how the process starts with a positive model (wax), creates a negative mold (sand), and results in a final positive object (bronze). Pay attention to the 'chasing' or finishing steps at the end, which are manual.
Now, let's contrast that with striking. This process doesn't use a mold in the traditional sense, but rather two hardened steel dies that stamp a blank piece of metal with incredible force.
The 'US Quarter Minting Process' video provides an excellent look at the industrial mechanics of striking. While it shows coins, the fundamental process is the same for struck medals.
Please watch two key segments: Blanking and Upsetting (00:57 - 02:47): See how the raw metal is first cut into blanks, which then have their rims raised. This raised rim is a hallmark of struck pieces. The Coining Press (02:47 - 03:48): Observe the speed and force with which the dies strike the blank to impress the design.
The core difference is clear:
- Casting is a form-filling process: liquid metal fills a cavity.
- Striking is a metal-displacement process: solid metal is forced to flow into the recesses of a die under immense pressure.
This fundamental difference leads to a cascade of non-negotiable design constraints.
The Critical Constraints: Undercuts and Draft Angles
For a medal to be successfully produced, it must be able to be removed from its mold or die. This brings us to two of the most important rules in medallic design.
Undercuts
An undercut is any part of your model that would prevent it from being pulled cleanly from a rigid, one-part mold—think of a hook shape.
In your sculpture work, you know that undercuts can be managed with complex, multi-part molds or, more commonly, with flexible molds (like silicone). The same is true for cast medals. A silicone mold can flex to release a model with undercuts, offering significant design freedom.
Struck medals, however, are a different story. The steel die is completely rigid. After striking the metal blank, the die must retract. If there is any undercut in the design, the newly formed medal will lock onto the die, causing a catastrophic failure in the high-speed minting process.
This brings us to the first "undeniable truth" of striking.
Undeniable Truths of Minting: #1 No Undercuts, #2 The Devil’s In The Bevel.
The author of 'medalblog', D. Wayne Johnson, provides a direct and practical explanation of these rules in a post titled 'Undeniable Truths of Minting'.
Please read the first half of the article, from the beginning down to the line, "[Undercutting is a sculpture technique...made by rubber or flexible molds.]". Focus on the explanation of what an undercut is and why it's a problem for die-striking.
The rule is simple: Die-struck medals cannot have undercuts.
Draft Angles
If undercuts are forbidden, how do we ensure the medal releases cleanly? The answer is with a draft angle (also called a bevel or taper). This means that all vertical surfaces of your relief must be sloped slightly.
For cast medals made in rigid molds, a small draft angle is necessary. For flexible molds, it's less critical but still good practice. For struck medals, it is absolutely mandatory. The blog post you just read continues to explain the specific requirements.
Undeniable Truths of Minting: #1 No Undercuts, #2 The Devil’s In The Bevel.
Let's return to the 'medalblog' article to learn about the specific requirements for draft angles.
Please read the second half of the article, starting from the heading "All relief requires a proper bevel." down to the end. Pay close attention to the specific degree requirements for different die-creation methods (e.g., pantograph, computer-generated).
The key takeaways on draft are:
- All relief elements in a struck medal must have a positive draft angle.
- The required angle depends on how the die is made, but a minimum of 15°-20° is a safe rule of thumb for modern digital workflows (CNC milling).
- When creating your 3D models in Blender, you must ensure every vertical surface, including the sides of lettering, adheres to this rule.
The Impact on Relief Depth
The production method also dictates how high your relief can be. Casting is generally more forgiving, while striking imposes strict limits.
Let's explore the terminology of relief as it applies to numismatics. You're familiar with bas-relief (low relief) and high relief from your sculpture background. In medallic art, the terms are used slightly differently, which can be a source of confusion.
Relief - Newman Numismatic Portal
The 'Relief' article on the Newman Numismatic Portal provides detailed definitions and, most importantly, clarifies the difference between sculptural relief and medallic relief.
Please read two key sections: Start at 'Types of relief.' Read the descriptions for High, Medium, Low, and Very Low Relief. Note how the presence of undercutting is a key differentiator. Then, jump to the section 'Relief in medallic art.' This part is crucial. It explains that what is called 'high relief' for a medal is still technically 'low relief' in sculptural terms because it lacks undercuts.
Here's the breakdown:
- Casting: Because flexible molds can accommodate undercuts, cast medals can achieve what is considered true sculptural medium relief. The relief can be quite high and complex.
- Striking: This process is limited to sculptural low relief (bas-relief) and very low relief (stiacciato or coin relief). The metal in the blank must flow into the die cavities. Very high relief would trap gas, create massive stresses on the die, and require an impractical number of strikes and annealing cycles. Even a "high relief" struck medal is still a form of bas-relief without undercuts.
Summary: Cast vs. Struck
Let's consolidate these requirements into a simple comparison table. This is the core knowledge of this lesson.
| Feature | Cast Medals | Struck Medals |
|---|---|---|
| Process | Molten metal poured into a mold. | Solid metal blank stamped between dies. |
| Relief Height | Can be relatively high (sculptural medium relief). | Must be low (sculptural bas-relief or lower). |
| Undercuts | Possible with flexible molds (e.g., silicone). | Forbidden. |
| Draft Angle | Recommended, but can be minimal with flexible molds. | Mandatory. A positive draft of ~20° is a good target. |
| Edge Detail | Often shows signs of a sprue being removed; can be irregular. | Typically smooth with a raised rim ("upset"). |
| Fine Detail | Good, but can be softer depending on the mold and metal. | Can be extremely sharp and crisp. |
| Artistic Freedom | More freedom with form and height. | Heavily constrained by technical rules. |
Conclusion
You now have a clear framework for understanding the fundamental production constraints of medallic art. The path you choose—casting or striking—has profound implications for your design. A struck medal is an exercise in disciplined illusion, creating beauty within a rigid set of rules. A cast medal offers more sculptural freedom but may have a different aesthetic character.
Key Takeaways:
- Casting is a form-filling process that allows for higher relief and can accommodate undercuts if flexible molds are used.
- Striking is a metal-displacement process that requires low relief, no undercuts, and mandatory positive draft angles on all vertical surfaces.
- The term "high relief" in medallic art refers to the highest possible relief within the constraints of striking, which is still a form of sculptural low relief.
In our next lesson, we will build directly on this foundation. We will examine the traditional workflow for creating struck medals, following the process from a large-scale plaster model to the creation of a steel die via mechanical reduction. This will bridge the gap between your 3D model and the factory floor.