I am looking for a firm mattress made of natural materials, free of chemicals such as formaldehyde and fire retardant. Do you have any recommendations?
Hi echosong,
Welcome to the Mattress Forum! 
For such a short question, you really bring up quite a few topics, so I’ll do my best to provide some information on what you’ve asked.
The first thing is that what I focus on here is “how” to choose a mattress, and not “what”, so everything I provide here will give you the tools you need to make an educated and informed decision for yourself. The first place I’d tell you to start, as a general overall guide, is the mattress shopping tutorial here. It will provide the logical steps of how to select a mattress (in as much detail as you desire), and also help you to avoid certain items.
When people ask about chemicals, they’re usually referring to a mattress that is using materials that are a “safe” as possible, or at least finding something that is “safe enough for me”. I know it can be confusing discerning between “natural”, “organic” and the different certifications for testing for harmful substances and VOCs.
There is more information about the three different levels of organic certifications in post #2 here and some of the benefits of an organic certification in post #3 here. There is more about the different types of organic and safety certifications such as Oeko-tex, Eco-Institut, Greenguard Gold, C2C, and CertiPUR-US in post #2 here and more about some of the differences between organic and safety certifications in post #2 here. There are also some comments in post #42 here that can help you decide whether an organic certification is important to you for environmental, social, or personal reasons, or whether a “safety” certification is enough. While I know this is a lot of information, if you take a few minutes to read through it I think it will help narrow down what is most important to you in a mattress.
As far as passing flame retardant certifications, fire retardant chemicals have never been mandatory. What is required is that a mattress pass the 16 CFR 1632 and 1633 fire regulations with or without the use of fire retardant chemicals (the method of passing the regulations isn’t specified in the regulations). The most common method used to pass the regulations is the use of inherent fire barrier fabrics that are either quilted into the cover or are wrapped around the inner materials of the mattress like a sock. There is more information about fire retardant methods that are commonly used in the industry in post #2 here and in post #4 here.
Regarding passing the more “stringent” tests for harmful substances and VOCs, you may wish to consider a latex mattress. Many of the members of this site offer such products, and they could be a good place to start your research. If you’re considering a mattress without any foam, while not a complete list, here are some options for mattresses using no foams.
Phoenix
Thanks for such detailed information! I really appreciated.
Hi echosong,
You’re most welcome.
Phoenix