Grandma's Soap: Recipe for Handmade Soap with Used Oils

Did you know that disposing of used oils can contaminate up to 1000 liters of water? Learn how to transform this resource into an ecological handmade soap and discover how to recycle used oils to protect the environment. In this article we teach you the step-by-step recipe to make soap from used oils, the essential safety measures and the importance of not throwing these oils down the drain. In addition, we invite you to learn more about our handmade soap making course to delve deeper into this fascinating world.

Introduction

Every year, thousands of households throw used cooking oil down the drain without knowing that just 1 litre can contaminate up to 1000 litres of water. In Spain, it is common to see oil being poured into kitchen sinks, without being aware of the environmental damage it causes: blocked pipes, flooding and pollution of rivers and seas, affecting aquatic life. According to the Aquae Foundation, disposing of oil can have catastrophic consequences for our water resources.

For this reason, today we bring you a recipe that allows you to recycle used oils to make the traditional "grandma's soap". With this technique, you will not only avoid pollution, but you will obtain a natural, biodegradable and environmentally friendly soap. In addition, if you are passionate about the world of soap making, you will be able to learn more about the use of natural oils. Our handmade soap course, where we teach you everything you need to make your own natural soaps.

The Importance of Recycling Used Oils

Environmental Impact of Used Oil

Pouring oil down the drain is a widespread but little-known practice. According to environmental studies, a single litre of oil can contaminate up to 1000 litres of water, affecting the quality of the resource and harming aquatic life. When oil is poured into kitchen sinks, it accumulates in the sewers, clogs pipes and can cause flooding. In addition, if it reaches rivers or seas, it forms an impermeable film that prevents the passage of oxygen, affecting biodiversity (MITECO).

Environmental data:
Studies indicate that oil buildup in sewer systems can increase the frequency of blockages and contribute to the pollution of waterways, damaging local ecosystems. You can find more details in the Municipal Waste document.

Damage to Aquatic Life

Oil pollution has devastating effects on aquatic life. When oil is poured down the drain, it forms a film that covers the surface of the water, preventing oxygen from entering and blocking the sunlight needed for photosynthesis. This oily layer directly affects fish, making it difficult for them to breathe through their gills, causing stress, skin damage and, in extreme cases, death. Imagine rivers and coasts covered in a thin layer of oil, where fish with clogged gills and damaged skin are a sad reflection of the human impact on nature (LinkedIn – Carolina Rouillon Gallese).

The Science Behind Soap: Saponification

Soap is a sodium salt resulting from the chemical reaction between a fatty acid and an alkaline medium. This process, known as saponification or alkaline hydrolysis, occurs when oil molecules (fatty acids) combine with caustic soda (sodium hydroxide) to form soap and glycerin.

Saponification Index

The saponification index (SI) indicates the quantity of caustic soda needed to saponify 1 gram of oil. In our case, we will use used oil (which can be a mixture of sunflower, olive, among others), and the approximate index is 0.136 g of caustic soda per gram of oil.

For example:

• For 1 gram of oil → 0.136 g of caustic soda
• For 100 grams of oil → 13.6 g of caustic soda

Basic Formula for Handmade Soap

To make a natural soap that can be used to wash clothes, dishes or floors, we will use the following basic formula:

• 100 g of recycled oil (used cooking oils)
• 13.6 g caustic soda (purity 99%)
• 21.6 g of demineralized water

Step-by-Step Recipe: How to Make Soap with Used Oil

Necessary Materials and Equipment

• Balance: Essential for correctly measuring ingredients.
• Nitrile gloves, protective glasses and mask: To ensure safety when handling caustic soda.
• Heat-resistant containers (glass or stainless steel): To mix the ingredients.
• Blender or mini-blender: To emulsify the mixture.
• Recycled moulds (PET or Tetra Brick pots): To pour the mixture and shape the soap.
• Stainless steel spoon or silicone spatula: To mix and pour the mixture.
• Filter: To strain used oil and remove impurities.
• pH Test Strips: To check that the pH of the soap is adequate after curing.
• Demineralized water
• Used oil: It can be a mixture of different types (sunflower, olive, etc.).
• Caustic soda 99% purity (sodium hydroxide)
• Vinegar:to neutralize accidental splashes on the skin while preparing the soap.

Security Measures

1. Personal protection:
   • Always use gloves, glasses and a mask.
   • Work in a well-ventilated area, preferably outdoors or in a kitchen with good air circulation.
2. Caustic Soda Handling:
   • Add the caustic soda to the water (never the other way around) to avoid violent reactions.
   • Make the mixture in a heat-resistant container, as the process is exothermic and generates heat.
3. Disposal of Used Oil:
   • Don't pour oil down the drain. Use a filter to remove solid waste and recycle the oil to turn it into soap, thus avoiding water contamination and clogged pipes.

Preparation of the Oil

Before you begin, filter the used oil to remove any food residue or impurities. This will ensure better quality in the final soap.

Step by Step Recipe

1. Preparation of the Lye

• Measuring Water and Caustic Soda:
  Weigh 21.6 g of demineralized water into a heat-resistant container. Carefully add 13.6 g of caustic soda to the water while stirring gently.
  Note: Add the soda slowly to avoid splashing, and always remember to do so with protection.
• Let it cool:
  Allow the lye solution to cool to a temperature similar to that of the oil (approximately 40ºC).

2. Oil Preparation

• Measuring and Heating Oil:
  Weigh 100 g of recycled used oil. Heat the oil slightly in a pot or in the microwave until it reaches approximately 40ºC.
• Mingle:
  Once both the oil and lye are the same temperature, pour them into a large container.

3. Saponification

• Emulsify the Mixture:
  Use a mixer or hand blender to mix the oil and lye solution for about 5 to 10 minutes, until the dough reaches a consistency similar to thick cream ("trace" phase).
• Pour into Molds:
  Pour the mixture into the recycled moulds. Tap the moulds lightly on the table to remove any air bubbles.

4. Soap Curing

• Rest Time:
  Leave the soap in the molds for at least 24 hours. Then, remove it from the molds and place it on a rack or ventilated surface.
• Cured:
  Allow the soap to cure for a minimum of 30 days. The longer it cures, the harder the bars will be and the better their pH will be regulated. During curing, check the pH with pH test strips; the soap should have a slightly alkaline pH, above 7.

Additional Tips for a Perfect Soap

Importance of Curing

The curing process is essential for the soap to reach its maximum hardness and quality. During this period, excess water evaporates and saponification is completed, stabilizing the soap's pH. Many soap makers recommend letting it cure for 45 to 60 days to obtain an optimal product.

Alternatives for Use and Conservation

• Daily Use:
  This soap is versatile and can be used for household cleaning, such as washing clothes, dishes or floors.

Conservation:
Store soap in a cool, dry place, away from moisture, to prolong its shelf life and maintain its properties.