Here are the main points to look for
This needs to be firm and flat and supportive, comfortable, and durable.
A foam mattress supported by springs (pocket or bonnell) will have a foam layer which can only be 2 cm deep either side of the springs.
N.B 2 cm foam top and bottom and 6 cm springs total 10 cm that is the British Standard.
2 cm is a thin layer and If this foam is not of a high grade dents and dips could appear after a few months wear.
Foam as a product will loose its strength if it is subjected to heat weight and damp and ironically that is exactly what a baby is!
For the foam to last several years and be supported by the springs without dipping the foam should be of a quality between 33 and 50 CMHR
It is vital for the foam to be firm. Soft cheap foam that gives no support could allow your baby to end up face down in a dip and suffocate. Keep this in mind when you read the next chapter about covers.
Here are more details on the construction of foam
Coir and Lambswool Base
The coir needs to be of 60 - 70% density then it will have a high tensile strength and be naturally springy.
One downside is although parents know that the mattress should be firm and flat some feel that the coir is too firm "feels like a brick"!
Putting a double layer of lambswool on one side solves this problem, It does not reduce the firmness but makes the initial lie more welcoming and offers the customer a choice.
2) Waterproof Protection
You have a choice where the waterproof easy-wipe-clean protection layer goes:- either over the cover or under it.
If you fit the waterproof protection over the cover, when you get the untimely accident you remove the sheet and wipe down the cover with an disinfection fluid like Milton, replace with a clean sheet and good to go
The downside of this system is this waterproof sheet if it is wipe clean will not afford any breathability to your baby or help with temperature control and will not reduce overheating.
If you fit the waterproof protection under the cover then you can have a cover full of air and that will offer temperature control, such as Coolmax® backed with Maxispace.
When the inevitable accident occurs remove the sheet and cover and wipe the protective layer with Milton or similar and fit a clean cover or topper, the downside of this system is you need to have a spare cover or topper (one for the wash)
Natural Mattress Cover
If you are buying a natural mattress because you do not want foam or anything that is not a natural product then you are asking for the impossible!
easy enough to make a natural base but when it coves to the covers you need a degree or waterproof protection which you cannot get from a natural cotton fabric.
There is a fabric called "Outlux" that is 93% cotton but has a have PU membrane in the middle (7%) to make the fabric waterproof. because the PU amount is so small it can be regarded as a natural fabric.
N.B there is an advantage in putting the Easychange Coolmax© and Maxispace system over the natural coir and lambswool base as you get full waterproof protection, full breathability and temperature control.
Here is the Easychange System
A video to show you the advantages of the Easychange