Advice on hemming silk?

[Physicsteacher]

I recently bought a used silk dress for a formal event, and it's too long so I'm going to need to hem it. My sewing skills are currently mediocre; I've made simple things like pajama pants and can handle basic maintainance and repairs, but I'm worried I may be a bit out of my depth working with such delicate fabric. The dress is a full length sheath dress with lining and a back vent.

DH has a sewing machine, but I suspect I need to hand sew this. What sort of stich should I use on silk? Is there a specific gauge of needle I should use? What else do I need to know to avoid screwing this up?

[Frankies Girl]

If you are really worried about screwing it up, go to a seamstress or tailor. A simple hem isn't going to cost you much.

I would attempt it myself but I'm an intermediate seamstress myself. I would do a basting stitch first, followed by a simple rolled hem with a straight machine stitch. VERY low iron heat to flatten the edge of the hem afterwards.

For a general idea: http://so-sew-easy.com/hem-sheer-or-lightweight-fabrics/

[bobechs]

Basting first is the right answer.  Use a contrasting thread, as thick as you have on hand, and a large-ish needle and take big, ugly stitches about a half-inch long, or even longer. Then iron the hem flat under a damp pressing cloth. and check that everything is in  proper postion for your final finish

You are just fixing the fabric in place and once the final stitching is in place go back and snip those big stitches in the middle with scissors and pull out the loose ends.  There may be holes left behind, but those can be ironed out with the same damp cloth.

[Frugal Lizard]

Pressing with the damp cloth is super important.  Not wet, just damp. No running the iron back and forth - press down and lift. Move pressing cloth into a new position, press down and up.  It is very easy to burn the fabric.

I would hem it by hand because I wouldn't be able to get the tension setting correct on my old machine for silk.  New (sharp) fine needle.  Put it on and have a friend pin it using a ruler measuring up from the floor to get a good straight line.  It would be best for you to be wearing the shoes that you will be wearing with the dress so that the hem is level.  If you pull the skirt up and let it fall and check the pins.  If the fabric is cut on the bias, it will be harder to turn the hem, so you just use lots of pins and use a hemming stitch that will allow a little movement. This will take some time but if a job is worth doing it is worth doing right (spoken in my gramma's voice). 

[hyla]

You're right that handstitching is better, I use my machine to hem basic things like casual cotton skirts and jeans, but nice dresses look a lot better with an invisible handstitched hem. 

My process for hemming delicate fabrics:
1. pin to desired length (ideally have a friend do this while you are wearing it), measure out 2"-4" extra for the hem, and cut off any extra length.
2.  Gently iron on medium heat to set folds, and if needed adjust the pins so that the fabric is even and smooth around the whole hemline
3. handstitch hem with invisible stitch (a youtube search will turn up lots of how to videos for this stitch, it's not difficult, just takes some time).  I usually space my stitches 3/8" to 1/2" apart when invisible hemming. 

For needles, I use whatever I have around that is reasonably fine.  I don't bother with basting, I feel that pins work fine for me to hold the hem in place as I'm sewing. 

If you are new to this, I would suggest saving fabric from the extra length you cut off the dress and practicing an invisible hemstitch on the extra fabric.  That way, if you do make any mistakes they will be on scrap fabric, and you can figure out what you are doing wrong and fix it before moving on to your dress.