It needs a bit of a study and a bit practise, but making a wig of weft mohair isn't as hard as you may think. I'll tell you my way of crocheting a wig.
For a wig for a doll with a circumference of 35 cm or 14" you will need appr. 100 gram of wefted mohair and a length of 105".
Top down crocheting:
For this wig I use as base for the crochet a brushed mohair yarn with thickness 5 and a crochet needle 3,5 (4 in the US) Start with a magic ring and crochet half treble (half double) till the ring is closed. In the next (second) row you add the weft to the work, behind the just crocheted cirkel and you crochet the weft together with the rest of the rows along the way, using the DOUBLE CROCHET (TREBLE). By using the double crochet in the next rows you avoid a too crowded crown.
In the picture I am a bit further but you can see how the weft is behind the crocheting.
You stick your needle through the weft, pick up the yarn and finish the stitch like usual
For those who want to see more than these pictures and like to see the wig & weft crocheting in action, there is a very good video available at http://fairywooldolls.blogspot.com.
In the second row you double your stitches by crocheting 2 times in every following chain stitch. In the third and fourth rows you crochet every other stitch 2 times, so 1 x 1, 2 x 1, 1 x 1, 2 x 1, etc. In the fifth row you swith to the half double stitch again, so the space between the rows become smaller and the wig will be more dense. To make a nice transition to the (practically straight) sides of the wig, you crochet every 6th chain stitch double. The rest of the wig you crochet as it comes without increasing. If you think your wig is ending too wide, decrease the last rows by crocheting some stitches together.
If you wish to make a wig of lighter hair or you want to use smooth yarn instead of brushed, I advice to use a thinner yarn like 3 or 3,5 (knitting needles as indicated on the yarn). Light wigs show more of the crocheting so when using a thin yarn, your stitches will be more delicate.
So summarized: you crochet your wig with half double stitches except for row 2,3 and 4 (those with double stitches) to avoid a packed and cluttered crown, you crochet the first 4 rows as a flat circle and you make a smooth transition to the sides of wig by increasing less than before. Always check during the process if the wig fits onto your dolls head; methods, used materials and crochet techniques can differ per person!
Bottom top method
When using a straight weft mohair, it is better (the hair falls nicer and jumps less) to start at the bottom of the wig and work your way up to the crown by decreasing in the same way as described above but vice versa ofcourse. You measure the circumference of the dolls head and you add 0,5"extra before you close the chain. You keep the weft also at the back of your crocheting. Decreasing at the crown is crocheting 2 or even more stitches together, depending on the need.
Good luck!
♥ Esther
Limited stock unfortunately
