I am going to guess here. My thought is that when the fabric is in the factory and is being wound on the bolts mice running overhead on pipes/beams/etc could drop into the machinery. As for twigs/grass/bird droppings, like a poster told us Bali batiks are dried outside, the wind blows stuff around and when they are folding mega yards of fabric...'stuff happeneds'. AGAIN this certainly happens rarely but it happens enough that I was glad to be aware of something I had never considered. Things like this does not bother me as long as I know to properly prepare my fabric. It's just like I wouldn't eat veggies fresh out of the garden dirt without washing them. I wash new dishes before I use them, new clothes before I wear them...

I always wash my fabrics to get rid of the chemicals and sizing. This is very important for your backing
which always shrinks in the wash. I just use warm water in my washmachine. After the fabric is dry and ironed,
then you will have its exact size. And then no problems with cutting it to fit your quilt top and filling.

wow. Never thought of that...will wash

I guess Im in the minority . I will continue doing what I have done before. No prewashing . If I had to prewash everything then nothing will get done. Life is too short to worry aout the "What ifs". We are becoming a nation of paranoid people. I can't worry about the unknown or the unseen. I continue to enjoy life.

In my case it isn't a matter of I can't be bothered with pre washing; it is a matter of preventing serious health problems. I am extremely allergic to the chemicals used in the manufacturing and processing if fabrics. I developed an autoimmune disorder and my dermatologist has traced it back to my quilting mainly the chemicals on the fabrics. Before we could figure out what the trigger was I was covered head to toe in itchy scaly seepy wounds. I looked like a burn victim. I was in constant pain and the itching almost drove me insane. I now will be on steroids and biological injections for the rest of my life. My joints are affected and I now have advanced arthritis. I am at increased risks of infections. We traced it back to the chemicals in fabric because right before my symptoms started, I had taken about a 5 year break from quilting and had not touched fabric in that time.

So those that don't suffer any allergies from the chemicals in fabric just know that I am very jealous of you. And count yourself very fortunate!

Okay, so I have never prewashed fabrics bought at a store, but have always washed fabric finds from garage sales or thrift shops. For those that do prewash, do you wash each fabric separately or do you wash similar colors together? Do you run a full wash or do a quick wash? I do ALWAYS wash a quilt as soon as it's finished as I'm not going to give away a quilt only to find out later that it fell apart or bled when the person I gave it to washed it. May have to consider prewashing though as I've noticed my hands have been starting to itch when touching fabrics in quilt shops lately.

I prewash to prevent shrinkage in the finished product and to remove excess dye - along with the chemicals and whatever might get into the fabric at warehouses and during shipping. My back door enters into my utility room so new fabric is in the wash before my coat comes off.

This.

In all the years I've been going to fabric stores and watching fabric get cut (my entire life, since my mom sewed everything I wore as a kid), I've never once seen a mouse in a bolt of fabric, or droppings, or seeds, or anything else. I'm not saying they are never there, but I'm saying it has to be a very very small percentage of fabrics that have this sort of issue. It's just not enough to make me change my habits at this late stage.

Oh, and as someone said, why is everyone so concerned about yardage, but not too worried about precuts? I wouldn't think there's any difference in how they are handled and that if there's a vermin issue, it would affect the precuts too. They come off the same fabric runs as the bolts, and stored in much the same conditions.

I'm a prewasher, because my eyes water whenever I'm in a fabric shop. I must be allergic to whatever chemicals are in the fabric. I'm glad I am, because at least 5 times out of 10, my color catcher comes out with color on it. Yesterday, I washed the fabric I bought from Thousands of Bolts - all blues which are notorious for running. The beautiful dark blue backing ran buckets, so I treated it with Synthrapol. The other blues were ok - surprising, because 2 were dark blues. I didn't measure the shrinkage, but I could tell there was some, because all the cuts were 1 yard pieces, and some were shorter than others after ironing! Doesn't matter, because I bought more than I need for the current project. I need more blues in my stash!