I just want your opinion on these portable little blacklights that are offered on E-Bay and About.com that claim to be able to see repairs, makeups and fakes. Have you ever used them or will you consider using them?
I would personally not consider a blacklight as my first investment.
I think the use for a blacklight depends on you porcelain buying situation and what kind of pieces we are talking about.
How a blacklight would be able to detect a fake is beyond me. Maybe while buying canvas paintings if someone have painted over an old signature and replaced "Smith" with "Rembrandt" you could see this better with a blacklight, but to most pottery and porcelain - I don't know.
Maybe a blacklight could be of some use to reveal repairs on pottery. Maybe also, if some of the decoration is repaired with ordinary paint or a part of the piece is replaced.
But most often a magnifying glass and your nose might be quite enough to spot repairs, since most pottery repairs smell of plastic and glue.
Regarding porcelain I believe that if you handle enough pieces you can feel the different touch of porcelain and paint. Most reveling - and even easier to feel - is the spray dust that often is all over the piece - except on the repaired area.
My way of checking pieces for damages is by just picking the piece up and turning it around a couple of times to feel if there is any "different" surfaces.
The repaired area could be smoother and "stickier" than the rest of the piece. (Sometimes the pieces are sprayed all over, to avoid this to be detected.)
At the slightest suspicion I hold it up against a really strong light. At an outdoor market, the sun might do. At auctions and fairs, there is often a spotlight somewhere around. Then you can see the repair as a shaded area clear through the porcelain. Even sprayed on paint will be visibly as a dark area.
The next thing I look for is hairlines. Really clean and new ones, that could have been made during the show, could be almost invisibly in soft light. Then you need a spotlight that can throw a beam of directed light along the surface. When this beam hits a hairline at a straight angle it becomes visibly.
If this is impossibly - the porcelain body could be too thick or the light might be to bad - I sometimes use a small needle and carefully scratch the area I suspect is repaired. This must be done very carefully so as not to ruin the repair that might have been quite expensive. It is also generally a good idea to ask the owner for permission before you do this. If he says no, then you know the piece is repaired. Steel are softer than real porcelain glaze and should not damage porcelain glaze more than ordinary cutlery would do.
I mentioned this just because a black light - even if it does work - as with any tool might give you a false sense of security.