Why egg yolk is used for ink?

Here's why:

* Egg yolk is a food, not an ink component. It's primarily made of fat and protein, which don't have the necessary properties for ink.

* Traditional ink ingredients include iron gall nuts, soot, gum arabic, and water. These components create a mixture that can be applied to paper and remain visible.

* Egg yolk could potentially be used as a binder in some ink formulations, but it's not a common practice.

Perhaps you're thinking of a different application? For example, egg yolk was sometimes used as a glaze for illuminated manuscripts during the Middle Ages. This was a technique for creating a shiny, protective coating on the artwork, not for writing itself.

Let me know if you have more information about the context of your question, and I can try to give you a more specific answer.