When I mentioned the concern about inks "going bad," I was referring to potential changes in consistency, color, or performance over time that might render them less suitable for writing or even potentially harmful to fountain pens. While it's heartening to hear that your older inks have remained usable, it's also worth acknowledging that not all inks may age as gracefully, for example modern inks that may have been formulated with planned obsolescence in mind, to ensure consumers keep on replacing old inks with new ones. They may perhaps use less concentrated/less effective biocide on purpose or to adhere to everchanging manufacturing standards.
Your mention of iron gall inks changing color but remaining usable is a valuable insight. It highlights the resilience of certain types of ink formulations and underscores the importance of understanding how different ink compositions may react over time.
Ultimately, the goal of discussing ink preservation is to share knowledge and best practices to ensure that fountain pen enthusiasts can continue to enjoy their collections for years to come. Your firsthand experience with vintage inks adds a valuable perspective to this conversation, and I'm sure many members, myself included, would appreciate further insights or advice you might have on preserving and caring for fountain pen inks.