The main difference is that earth pigments are broad‑band absorbers with irregular particle sizes, so they scatter light and give a subtle depth. Synthetic pigments can be engineered to have precise absorption peaks and higher optical densities, which makes colors appear more vibrant but also changes how light penetrates and reflects. That means a synthetic red can look brighter at a glance, but its lightfastness and the way it interacts with a glazing layer may differ from a natural cinnabar or vermilion. In short, the shift gives artists more control and stability, but it also alters the optical behavior of a painting in ways that require careful study if you want to replicate the visual effects of the old masters.

You’re right to focus on the optical side; the real question is whether the engineered purity of synthetic pigments can mimic the complex scattering that earth materials naturally produce. If you take a glaze of a modern ultramarine and apply it over a ground of synthetic umber, the light will still hit a uniform surface, so the subtle gradations of the masters’ wet‑to‑dry transitions may be lost. To match that effect, we’d need to study the microstructure of the old grounds—particle size distribution, binding media, and the way they were layered. Only then can we formulate a “new rule set” that recreates the same play of light while retaining the stability of contemporary pigments.

That detective work is exactly the right mindset—measure the particle spectra, quantify the binder’s refractive index, then compare the scattering coefficients. Once we have the numbers, we can adjust modern pigments to match the optical depth of a century‑old ground. It’s meticulous, but the payoff is a reproducible process that preserves the masters’ visual language.

Exactly, the key is to let the data guide us so the modern studio can emulate the centuries‑old effect without losing the essence of the original technique.