Is there much of a risk of a person over-stretching a fur? If so, is it primarily caused by using too big of a stretcher, or by leaving it on the stretcher too long?

Yes overstretching fur costs money.This is moreso a factor for pelts coming on to prime.Mark your size lines on your boards and pin in between the lines for cased fur and on the line for beavers if using the sizes provided by the auction house.
Also on cased fur,pulling the back legs down to pin thins fur in the flank.Pin the skirt straight across from the base of the tail first,between the size lines on your board,and then just pin the back legs where they hang.Pinning between the lines willguarantee that the pelt will remain in that size parameter after removal from the board and subsequent shrinkage.
The size lines for beaver are 2-3 inches over the size parameters so pinning on the line in the case of beaver will allow for shrinkage.

In my opinion with out using some type of apparatus you can't by hand over stretch a hide.

Here's a little test you can do. Take a coon and pin It out so It just snugged up, check It out for fur density. Then remove the pins and stretch It as far as you can. Then look and see if there any visible voids In the fur. Do the same with any fur out critter and get back to us with your findings.

There Is a lot of elasticity In a green hide so get as much length as you can. A inch one way or the other can put you In different grade. And that can make you money or you can lose money.

In my opinion leaving rats on wire for more then 3 days Is going to cost you money. Wire acts like a spring and Is always pushing back as a skin dries. This will thin that rat hide. And I can tell you that a thin hide with that crinkly sound to It Is going to get you down graded. Take a rat hide done on wire and one done on wood and you can tell the difference between the too even if your blindfolded. The rat on wood will be very supple and be a lot heavier to the feel.

I generally agree with that statement with the exception being beaver. When a beaver has been tacked out such that the nails bend over and the hide rips away from them when drying, the pelt has been over stretched. To avoid this, when the nailing is complete and the pelt lifted up on the nails, for drying purposes, the centre of the pelt should sag to the board.

A well known fur processor told me that on beaver he takes a pair of vise grips and pulls till he gets to the next line. I think that's going a little to far. So far I have never had the hide tear off the nails so I must be doing It Ok.

The only real "problem" I ever had starting out was with wooden stretchers as there is not any give in them. Wire forms flex enough that it is almost impossible to overstretch them, because the fur will tear usually in the wire hooks first.
It is just common sense in stretching fur in you can see you are over doing it. I had more problems with fox than anything.

I wonder, then, why Groenowald favors wire so much more than wood.

Because your rats come out bigger on wood. So Instead of buying a 17" rat he can buy a 15" rat that was stretched on wire. Remember size matters.

He did say that wire stretched rats stack up better. Big deal.

Means double the price...........................on marten. just look at the last auction from xl to xxl size is double the price........so an inch means a lot of dollars

Besides it not stretching it's FORMING.
Beav your right about the above post.

Stretching vs forming...hmmm...I guess I've not given much consideration to the difference between those 2 different terms. Everyone always seems to call it stretching from what I've heard.

Only if they don't group them in the same lot like NAFA is doing with sda's I remember back in the late 90's early 2000's when they lotted all the XL's and XXL's together. Good for Canadian XL producers.

Canadian XXl producers
the xl is only 1/2 the price

the new bailing is with 200 skins now then it was 80.

Over stretching on wire for coyotes bows the sides in and makes for thin through the middle and fat on both ends. Looks like crap. Wood not an issue. Although the width has to be right at the base and can pull the belly to the edge and make the hide look thin in that area. Just doesn't look uniform. XL to XXL on cats is a $100 bill or more. I bought some wooden cat stretchers that had a fat head on them that cost an inch or more on a stretched hide. They should be tapered to a point. Had to sand them down. I told the builder and he said I was full of crap. I think it is possible to over stretch a cat. Makes the hair lay funny and thins the shoulder area out. Almost looks like a rub.