[CamperBob2]

That could be a good point, I suppose. Iron-based ink could have a similar effect. I'll have to see if there's a deck of cards around here to X-ray.

[BertoldVdb]

We tried this long ago at a university, the cards we had were entirely invisible to the x-ray.

I think the 'x-ray' table in the article works with IR cameras and illuminators under the table, and tablecloth that is slightly IR transparent.