There are so many different yarn bases, it can sometimes be very intimidating! I hope this info page will help to make it easier. :)
You will notice when shopping for yarns that they come in different weights. The thinnest yarns are sock, fingering, and lace weight. These are best, as the names suggest, for projects such as socks or thin lacey items. They are not appropriate for diaper covers or children's clothing in general. More medium weight yarns are DK, aran, and worsted weight. These will work for thinner clothing items, such as summer tops, and different diaper covers. Heavy weight yarns are called bulky or chunky. These also work well for diaper covers and sweaters. You may find them too thick for summer clothing, though it is still very possible to use them for that purpose (and vice versa with medium weights for winter clothing).
Wool is a very special fiber as it has many interesting properties. It is naturally antibacterial which helps it in not holding stink. It allows air flow (unlike plastic diaper covers) and adjusts itself to temperature in that it will keep you warm in cool weather but won't be too hot in warmer weather (providing you aren't wearing a super thick sweater or something ;)). And of course it is water resistant which allows it to work effectively as a diaper cover. It can even hold up to 30% it's own weight in moisture. Lanolin is a naturally occurring substance in wool which adds greatly to it's water resistant properties. Therefore wool covers should be washed in a mixture of lanolin and mild soap (as the amount of lanolin in the wool will reduce over time, and must be replenished).
All 100% wool yarns must be washed by hand in lukewarm water, then air dried, otherwise there is a risk of felting. Felting means that the wool fibers blend together with each other and the garment loses it's stitch definition and stretch. It generally also shrinks. It occurs only with sudden temperature changes and excessive agitation. It will be harder to felt tougher yarns, but you still should always exercise caution. Watch your water temperature carefully and don't scrub hard with soap bars or twist the wool upon itself to remove excess water. Gentle is the key. :) The best way to remove extra water before laying the garment out to dry is to carefully squeeze out as much as possible and then briefly roll it in a towel like a burrito. Make sure to reshape the item on a flat surface before allowing it to dry or it could take on a funny shape.
The following yarns are more suited to smaller, non-mobile babies, as they tend to pill or felt more easily with rigorous use. However they are extremely soft.
- Blue-faced Leicester (BFL).
- Organic gaia
- Licorice twist or merino/aran twist - this is an interesting yarn which has one ply of superwash that takes dye more deeply than the rest, creating a swirl effect.
- One-ply or 3-ply uruguay merino (Purewool)
- Australian Merino
These yarns are more suited to older, mobile babies and children as they are less prone to pilling and felting. They are not quite as soft as the yarns mentioned above but they are by no means scratchy either.
- Cestari Fine or Superfine merino
- Mountain Meadow Rambouillet (MMR) - my personal favourite.
- Blackberry Ridge Merino (BBR)
- Marr haven - this yarn is very hard to find.
These yarns can sometimes be more scratchy, though unfortunately may also pill. It depends on the supplier. They will still work effectively as a diaper cover and tend to be on the cheaper side. They are most likely not good for newborns as the scratchiness may irritate their skin.
- Peruvian Highland
- Paton's - can be found in local stores such as Michael's.
- Peace Fleece
Other types of wool include cashmere, alpaca, and angora (extremely soft), and mohair (can be scratchy).
Superwash yarn is wool yarn that has been treated by a process which allows it to not felt. Therefore it can be safe to machine wash, though I would still not recommend this with hand knit garments. However, you will not need to be as careful with water temperature or agitation while washing. This yarn is not really appropriate for diaper covers as it does not retain lanolin. It may work for one time use between washes in the same way that fleece covers are used. It is wonderful for sweaters, hats, or pants, etc for older children who no longer need diaper covers. It still retains many good qualities of regular wool yarns that do not exist in synthetic yarns (such as acrylic).
Cotton is another nice natural yarn base, but it does not work at all for diaper covers as it is an absorbent material. It is not very common as a hand dyed yarn because the process for dyeing takes a different type of dye and is more time consuming than dyeing wool yarn. Cotton does tend to be soft and it is a good alternative to superwash yarns if you do not need it for covers. It can usually be found at local stores. There is also no risk of felting cotton yarn.
Acrylic yarn can be found generally at local stores. It is a synthetic base and therefore does not offer the same qualities of air flow or temperature control as wool. It can work well for those on a budget as large quantities do not tend to cost much. However from my understanding, though I have never used it this way, it works much the same as superwash as a diaper cover - a one time use and then it will need washing.
I will update this page with more information as I learn more about different yarns! I regularly use most of the types of yarn listed above (particularly the wool and superwash yarns). I am always open to knitting any type of yarn for customs.