That’s a very good question but it’s easy to answer when you know what hair fibre your doll has. In general there are only three hair fibres that were ever used by Mattel. You have Saran hair ( early dolls have until mid 1980s used widely), Nylon hair ( some 80s dolls have Nylon hair, very shiny, you find it mainly on MLP My little Pony) and Kanekalon hair ( mainly used in the 90s for Barbie dolls with giant amount of hair).
What you can do with every doll head is wash the hair to revive it. It’s artificial hair so nothing can really happen except that Barbie dolls produced in the last 15 years could loose their hair because they are glued in with resin or a glue, anyway something very gluey and stick but not resistant to heat. It seems that Mattel glued the hair in the head after they bought new machine which work automatically. It seems that the new machines don’t need a worker anymore. This a way of cost saving but what about the quality? Do you know that the glue comes out of the head after you debox the doll and comb it? Some modern collectors say the hair has a feeling like wax, not very pleasant. The original way how dolls got their hair is this one…
Here you have a worker but no glue. You can imagine it like a sewing machine for doll hair. Your thread is the hair. By the way these machine are expensive up to 3000 € and more in used condition.
But back to the topic now
The easiest way to revive the hair is by washing it. Treat it with conditioner instead of soap. I show you in one of my videos how I do this. The procedure is a little labor intense but it’s worth the procedure because the hair is always looking much better than before. What these sewing machines for doll hair do is this here
So basically due to the pattern inside the head that the machine made nothing will happen when you treat the doll with the conditioner. The hair pattern done by the machine is secured inside the head. You must proverbial cut the hair very short to be able to destroy the pattern. This here is my way to revive the hair …
This method also works for shorter hair and is a very good method to save hair so that your doll doesn’t loose so much hair. This method also works for Saran and Nylon hair but these fibres don’t tend so much to tangles. By the way the doll in my video has Kanekalon hair and I think this fiber was used mainly in the 1990s because it was cheap.
So I hope I’m able to give you a good idea how you can treat the hair of your doll to revive it.