Visibly, legibly and indelibly

There are three criteria for markings

In various pieces of legislation you will find the passage: “the marking shall be affixed visibly, legibly and indelibly”. It makes perfect sense that markings remain present and usable throughout the entire lifecycle of the product.
But how can you show in a practical way that all three criteria are met?

1. Visible

The first criterion is what you expect, namely that you can see it. Markings must be easily accessible for all parties, typically the user. It means ‘in sight’ or ‘visible from the outside’ for instance on the back of a product. Markings shall not be hidden behind lids or parts of the product.

2. Legible

The second criterion is a bit more tricky, because legible might not be the same for each person. Marking or inscriptions can be shown in various ways, colours, solid or hollow, fonts. A practical approach is to translate legibility or readability into size. For instance the minimum height of CE marking is 5 mm to ensure legibility. The minimum size of symbols, markings and instruction is very often stated in the applicable legislation or standard.

3. Indelible

The third criterion is perhaps the most difficult one to check. Indelible means that it shall not be possible to remove markings ‘under normal circumstances’. 

A practical method to check this is the “rub test” with water and petroleum spirit, for instance according to the procedure as mentioned in standard EN 62368-1:

“The test is conducted by rubbing the marking by hand without appreciable force for 15 seconds with a piece of cloth soaked with water and at a different place or on a different sample for 15 seconds with a piece of cloth soaked with petroleum spirit. After each test, the marking shall remain legible. The marking label shall show no curling and shall not be removable by hand.
Petroleum spirit is a reagent grade hexane with a minimum of 85% n-hexane (CAS 110-54-3).