Ink, metal, chalk, or dye on paper, it is the paper that holds the “stuff of” and it also is the source of the light.

Albumin Printing: https://www.bostick-sullivan.com/product-category/alternative-process-kits/albumen-printing-process/

Or, albumin paper: https://analoginside.com/en/product/20-papeles-albuminados-85×11-216x277cm/

Unsensitized photo-paper: https://www.fotoimpex.com/brands/adox-art-baryta-241×305-cm-95×12-inch-50-sheets.html

Reflection requires smooth surface — smoother is more reflective. This provides higher “saturation” — the Dmax. The topmost covering produces the first and last light scatter. In photography of the Silver Gelatin type, this is Gelatin. Albumin has also been used. Many colloids have been tried in search of the perfect holder; that stuff with perfect properties — lasts, passes liquids, blocks gasses… No perfect single thing has been found — we live with a suited to purpose answer.

The ‘goo’ adds color– albumin is different color than gelatin. They age differently. As matter of cost,the albumen silver print uses more silver than the silver gelatin print

Make it smoother; get higher density. Gelatin seems our easy, cheap solution. That is what we have used at industrial scale since the 1930s. Keep the emulsion thin and smooth to achieve higher density.

The secret to Dye Transfer’s intense color isn’t due to the matrix nor the dyes but to the paper, one of the most protected secret of the process. That was the item causing Kodak to stumble in the 80s. The process of the mill was little understood by much of the Kodak elite.

Additional to coating is HOW you coat; the tool, coater, as well — how many coatings. Making the emulsion means controlling crystal size, dispersion and “layer” —