Well, some yarns are not so accommodating. I'll start by saying I have completely given up on anything single ply. If you use them as warp threads, my hat's off to you. I find they pull themselves apart too easily and the damage is difficult to hide.
Sometimes a 2 or more ply yarn can be fragile. Overall the yarn can hold up to the stress of being woven. But the constant rubbing of shed changes, and the on again, off again tension of advancing the warp can sometimes take it's toll.
If the warp breaks altogether, it's pretty obvious you have to fix it right away. Simply take a length of yarn and tie it in to replace the damaged area. I would suggest making the knot as small as possible to keep a low profile.
Because it will be a small knot, it might not be very strong. Leave the tail ends on until right before you weave the row that will include the knot. The tail ends will be annoying during shed changes. Still, they are better (less annoying) than your knot slipping undone because you cut the tails too short.
Sometimes only one strand of a 2 or more ply yarn will break. If left alone, the weak spot will likely break before you get it woven in. Rather than cut it and tie in a new segment right away, I suggest placing a bit of tape on it. Here's why:
1. When you tie in a new segment of yarn, you'll almost never get the tension just right. Then you'll have one warp that's off. If it's too tight, you're adding more stress to an already weak thread and risk pulling your knots loose. If it's too loose, well, that's just annoying. It can also cause bubbling in your weaving.
2. By using tape, you might be able to prevent having to ever splice in a new segment. The tape reinforces the weak spot until right before it is woven in. If you can avoid knots, you lessen the risk of an uneven look to your pattern.
How to Use Tape for Warp Repair
I use packing tape because it's wider which I find easier to handle for this application. Scotch tape is usable as well. The tape should be clear so you can see the warp through it. This will help you not cut the warp when you cut off the tape.
1. Cut a length of tape long enough to cover the damaged area.
2. Holding the offending warp out from the others, place the yarn up the middle of the tape. At this point you should have a piece of tape still flat with the yarn running up the middle of the sticky side.
3. Fold the tape sticky side in so that the edges line up (any hangover can be cut off so the sticky does not damage the other warps). Do not press all of the tape flat! Only press the edges of the tape together so that it will stay in place. The center fold, around the yarn, should have a bit of air space.
4. Until the tape enters the working space, I leave it at step three.
5. (Optional) Once the tape passes the heddles and enters the working space you can trim the tape down. The picture below shows the tape after trimming. Trim so there is barely any stuck together area. If you cut into the air space, you will be cutting the tape off.
Reducing the flap width will make it easier to change sheds. I wait until the area is in the working space because the tape is not as strong at this point and I do not want it to come off too early. It's fine if you do not want to trim the tape. The flap gets in the way when changing sheds. It is still workable though so it's up to which feels better, less annoying shed changes with stronger tape or weakened tape that will allow you to work faster.
6. Once the tape gets right up to the woven area, carefully cut into the air space. You can also try to pry open the stuck together part of tape.
7. Carefully remove the tape. Some fiber is bound to come off, but hopefully not so much that the warp breaks.
8. Carefully weave in the damaged area.
Here's a photo of the result after I wove in the above damaged warp:
The damaged warp was in the middle section. It starts about an inch above the bubbled area on the left border.
That bubbled area kind of amuses me. I was fussing over the damaged warp which I noticed shortly after warping for the entire weaving. All that time I didn't even notice this other damaged thread until it created that first bubble. The bubble is one ply that broke and then got squished up the length of the other ply. The white bits for three rows after the two bubbles shows the single ply area.
This shows what you risk your pattern looking like if you do not catch a damaged area and smooth out the broken ply. After I take this piece off the loom I will use tweezers to try and pull out the second ply a bit. There will still be the white, but I like this better than having knots in my warps.
Advancing Fragile Warps
Many yarns can just be tugged and generally abused which includes getting yanked through the string heddles. For fragile yarns, the string heddle can catch one ply and bunch it up, possibly to the point of breaking, while the other ply travels through.
Normally I just shove the heddles up the warps. One hand pulls the woven area toward me. The other hand pushes the heddles. When doing this the heddles are pushing up an incline as seen below:
To reduce rubbing on fragile (or sticky) warps I will used two hands. With one hand I hold the warps in a way so they are level before and after the heddles. The other hand then pushes the heddles back. Normally I hold the warps flat with my right hand but I couldn't figure out how to hold the camera with my left hand.
So here my thumb is pulling down on the warps beyond the heddles. My fingers are pressing down on the bottom shed, keeping it out of the way. For less sticky threads, the reduction of friction is more readily visible. The heddles will go limp as you take all tension off of them. This reduces pilling, and therefore damage to the warps.
As ever, feel free to ask questions if I've totally confused you. I'm hoping to make a lot of these tips and whatnot into videos but, well, time. and camera shy. and time. So ya, someday. Until then, Happy Weaving :)