Your question is about "what to look for" (a good way to phrase a question since no lens is ever "best" and that becomes a subjective question where you'll find a wide variety of opinions.)
I'll try to describe a few ideas and I'd like to start with what it means to have a "macro" lens (how are they different than a non-macro lens).
The ability to bring a subject to focus at extremely close distance. While definitions of "macro" vary, you'll find a school of thought wherein the lens should be capable of producing a 1:1 scale image. This means the image projected into the camera sensor is as large as the subject in real life. Your sensor is roughly 15mm tall by about 22.5mm wide. Think of a coins such as a US penny... about 19mm diameter. It would barely fit on the sensor in the "wide" dimension and you'd have to crop off a few millimeters in the narrow dimension. Clearly your pottery is much larger and doesn't require a lens with true "macro" capability.
We tend to think of the plane of focus as being a "flat" field. But it's actually moderately curved. This means if you capture of a photo of a subject which is actually flat (think of a painting), the edges and corners of the image may be a tiny bit soft even though the center of the image may be tack-sharp. This is useful for flat subjects (such as painting, drawings, etc.) but your pottery isn't "flat" ... so this isn't really a situation where this attribute would benefit your work.
True macro lenses usually DO go out of their way to have better resolving power than a typical lens. This is an area that may be useful for your work.
Canon does make an EF-S 60mm f/2.8 Macro USM. This lens is not too expensive, it is a true macro (capable of 1:1 scale close-up images -- not that you'll need it for your work) and it is particularly sharp (arguably one of the sharpest in the EF-S series ... if not the sharpest in the EF-S series. I previously owned one of these lenses but donated it to a nephew since my camera bodies need full frame lenses (EF-S lenses only work with camera bodies that have APS-C size sensors.)
I also own the EF 100mm f/2.8L IS USM Macro. This is an impressive lens ... roughly 3x the price of the 60mm macro. While I do appreciate the 100mm macro, the 60mm macro will easily compete with the 100mm macro with respect to its ability to resolve fine detail. For your particular application, I do not feel the 100mm offers any advantage over the 60mm (unless you plan to upgrade to a full-frame sensor camera body someday.)
A 100mm focal length lens will require moving the camera a bit farther from your work. This is fine if you have the space.
I should add that you can find zoom lenses that will manage to work "macro" somewhere into the name. These lenses allow closer focusing but never true 1:1 scale (often 1:4 or perhaps 1:3 scale) and they do not usually share the attributes of their true macro siblings.
Other lenses can do amazing work but you will want a solid platform. I did notice that one of your images exhibited symptoms of vibration (the image was blurred only in the horizontal direction). This indicates that possibly the camera was shaking after having pressed the shutter button. The solution for this is to either use a remote shutter release or use the camera's timer to delay the shot so that the camera has a few seconds to stop shaking after you press the shutter.
I should add a "generalization". I am cautious to add this information since, while it is generally true of many lenses, it is not always true. Please consider this as I offer this information: Lenses are generally not sharpest at extremes. The lowest possible focal ratio the lens offers will usually not produce the sharpest results. Often stopping the aperture down about 2 f-stops will noticeably improve sharpness. E.g. if you have an f/2.8 lens... consider shooting at f/5.6. If you have an f/4 lens... consider shooting at f/8. You really would need to carefully test the lens to find the best results.
I think most lenses with a moderate focal length (50mm would be good) will produce a fantastic image. Stop down the aperture (f-stop) to produce a generous depth-of-field. Make sure the camera is on a solid platform and wont shake.
Your lighting should be fine... but you might consider a few tweaks (if you're up for it). One of your plates had a strong reflection in the center. Simply adjusting the angle of the light (imaging the plate is a 'mirror' ... would the mirror reflect the light source?) This is something we always have to consider when photographing shiny surfaces. A circular polarizer can help you reduce reflections (also... you can attach polarizing film to the light source. Although it looks like you have a rather large light source).
Sometimes reflections can help provide an accent ... revealing the shine on the glossy surface of your art. An accent light off to one side to provide a reflection that you can control (control the location and limit the size) can be a benefit here. The book Light Science & Magic (by Fil Hunter, Steven Biver & Paul Fuqua) is a often recommended for help in understanding the lighting concepts.
To sum up
- Even Canon's 50mm f/1.8 is optically very sharp.
- The EF-S 60mm f/2.8 Macro USM is particularly sharp.
- The EF 100mm f/2.8 Macro USM (there is also the "L" series with IS) is also extremely sharp but will require more working distance between lens and subject.
You need a solid mounting platform for the camera to avoid vibrations ... use the self-timer or a remote shutter release.
- Stop-down the f-stop moderately... consider 2-stops from wide-open as a good starting point and evaluate the image quality to find out what works best for your specific lens.
- A few improvements to lighting will be helpful (the only reflections on your art should be the reflections you deliberately want to show in order to reveal the shine/glossy nature of your subject.)