What Containers Can I Safely Use For Cold Process Soap Making?
Most containers can't be used in cold process soap making. This is because sodium hydroxide (NaOH) is incredibly alkaline and it will make some materials disintegrate. In addition to the possibility of a vessel dissolving or exploding, toxic compounds could end up in your soap. Lye water also becomes hot. The vessel you select will need to be robust, heat resistant, and one that can tolerate alkaline solutions.
With this in mind, let's examine which containers to purchase, and which ones to shun.
Lye and raw soap batter should never come in contact with:
- Aluminium: Aluminium reacts with lye to produce hydrogen gas. Hydrogen is combustible, so mixing lye with aluminium is risky.
Glass is mainly produced from soda lime. Sodium oxide (soda) is blended with calcium oxide (lime). Most of the worlds' glass production is soda lime based, including lighting and windows. However, this type of cheaply produced glass should be avoided in soap making because it is a soft glass with low shock resistance. It fractures readily. It would not be able to deal with lye water. Over time, sodium hydroxide forms sodium silicate over time when exposed to glass and becomes damaged.
How about tempered glass?
Tempering is a process which can make soda lime glass harder. The glass is heated to a high temperature, then promptly cooled. This generates a stronger glass that is three times stronger than non-tempered glass. Instead of shattering into shards, it crumbles when broken, just like glass used in car windshields. If there is a weakening, imperfection or chip, over time, the glass will shatter as a result of thermal shock. Once damaged it must be disposed of, or replaced, just like windshields with cracks. Lye water will etch and weaken tempered glass over time until it breaks.
If you only want to mix the oils and lye together and bring them to trace, tempered glass is fine though. This process occurs in a relatively short timeframe, just enough for mixing and pouring after trace. But it is not recommended for lye water.
How about Pyrex?
Pyrex containers were once made of thick, tempered borosilicate glass. They were more resistant to chemical reactions and thermal stress then. Think back to your school science lab days. Borosilicate glass is a hard glass that doesn’t shatter. It is composed mainly of silicone dioxide and boron. It is also chemically resistant and thermally stable. In 1998, Pyrex stopped using Borosilicate glass when the brand was licensed out to other manufacturers. Corelle Brands, which became responsible for US production, changed the glass composition to soda lime glass and there were reports of exploding glassware. There have been various recent class actions against Corelle Brands which supplies the U.S.A, South America and Asia. In Europe, the Middle East and Africa, Arc International manufactures the Pyrex brand, and they still uses borosilicate glass.
PET, PC, nylon, PS#6 or ABS should never be used when working with lye solution or raw soap batter.
However, you can use Polypropylene plastic for your lye water. PP#5 (recycling code 5) is the only plastic that is heat proof and chemical resistant. Check the base of the plastic container for the stamp.
HDPE#2 (High density Polyethylene) is a plastic (recycling code 2) that can store lye powder or cold lye water in. But it is not heat resistant so cannot be used for hot lye water.
You can safely use stainless steel for lye water!
Stainless steel has a melting point above 1400 degrees C so it won't melt or leach.
Polypropylene (PP#5) has a melting point of 160 degrees C. It is also more affordable than stainless steel, and it is microwave-safe.
Those are your best two options!