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The Iron Furniture Guide
Forging & Casting, Pros & Cons, and Maintenance Tips
Iron is the embodiment of strength and resilience, which is why it is our choice metal for many of our pieces. Few materials rival iron when it comes to forming a strong, durable base. Not only is iron a great material due to its characteristics - it can be produced in an eco-friendly and sustainable manner as well! This article covers everything about the iron we use - from the types of iron and their sources, to maintenance tips for our bases.
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Why do We Use Iron for Furniture?
Iron is our choice metal for many of our pieces. This is due to the strength inherent in both cast and forged iron alongside its naturally neutral color scheme.
Cast iron is actually a carbon-iron alloy, while forged iron is made up entirely of iron. The carbon added in the cast iron process increases the molten iron's strength when cooled, while forging iron allows for the retention of the iron's natural grain structure, leading it to be a stronger material for furniture than any other alloy on the market, including bronze and aluminum alloys.
Iron has a naturally cool and neutral color. This makes it incredibly versatile as the color palette matches a variety of styles. The main styles that influence our designs for iron furniture are mid century modern, rustic modern, and industrial modern, as the elements from these styles compliment the natural colors coming from the materials we use.
Types of Iron
We use two main types of iron: Cast Iron and Forged Iron. These two types differ due to their creation process.
Cast Iron is a type of iron formed by pouring molten iron into a cast, letting the metal cool & harden, then extracting the cooled piece to for finishing. It's most often mentioned in relation to cookware - as cast iron pots & pans are known to last longer and retain heat better than their stainless steel counterparts.
There are several methods used for casting iron. Among these methods is sand casting, which is an ancient process, evolving from the clay casting method used during the Shang dynasty (c. 1600 - 1046 BC). This method was first recorded by the Italian metallurgist Vannoccio Biringuccio in his book, De la pirotechnia(Concerning Pyrotechnics) in 1540. This method was adopted by the automobile industry when the Ford automobile company set a new record in 1924 by producing a million cars, consuming one-third of the total casting production in the US. This led to improvements in the overall efficiency of the sand casting process by improving several major bottlenecks in the production process.
Why We Use Cast Iron for Furniture
- Durable - Cast iron is the choice material for many automotive parts for a reason - it can withstand a ton of wear-and-tear. By using cast iron for our bases, we ensure that the piece can withstand anything life can throw at it - and much more.
- Tried-and-True Technique - Sand casting is a technique first invented in ancient China, and improved upon by metallurgists for thousands of years. By using a system that has been refined by dozens of generations of craftsmen, we not only pay homage to those who came before, but also guarantee that the piece will be of the highest quality.
- Eco-Friendly & Sustainable - After sand casting, the remaining scrap can be melted for use in the creation of the next piece. This ensures that nothing goes to waste. In addition to eliminating waste, our sand casting process exclusively uses materials which minimize the emission of greenhouse gases during production.
How It's Made: The Iron Casting Process
Iron casting involves creating a pattern for the mold, melting the metal, forming the mold, pouring in the iron, letting it cool & harden, and carefully removing it from the mold before finishing the piece. Each step in the process is elaborated upon in the below section.
Creating a Pattern
The pattern is a replica of the desired end product. This is what will be placed into the sand in order to form a mold into which the molten iron will be poured. We create wood patterns, as wood is the most eco-friendly choice, but patterns can also be made of metal, plastic, or plaster.
Patternmaking is one of the most important parts of the casting process, as this will determine the shape of the final piece. Precise calculations are needed in order to ensure that the different pieces will fit together when the entire product is assembled.
In addition to creating an external pattern, coremaking is also necessary for pieces which are internally hollow. The core is simply sand placed within the mold in order to cast the correct internal form. Cores are typically strong yet collapsible so they can be easily removed after casting.
Unlike forged iron, cast iron is first melted prior to molding. During this process, the iron scraps are heated to their melting point until the metal is in a liquid-like state. In order to make the iron strong and usable, carbon must be added in the right amounts. This addition of carbon serves to make the iron harder and stronger on an atomic level.
Forming the Mold
To create the mold, sand is first mixed with molasses in order to give the sand more structure. This sand mixture is then poured into a box frame, which will serve to contain the mold during the casting process.
Once the sand is placed in this frame, the wood pattern is placed into the sand and positioned just right in order to form the mold. Once the pattern is in the correct position, the remaining sand is poured in alongside flour and compressed in order to form the full mold.
Once this process is completed, the pattern is then carefully removed from the sand, leaving behind an imprint of the pattern within the sand, also known as the mold.
Pouring in the Metal
Before pouring in the molten iron, a gating system is set up. A gating system typically consists of a pouring cup and tunnels, or "gates", which funnel the molten iron into the mold cavity.
Once this system is set up, the molten iron is then poured into the cavity of the mold. Sand casting actually supports a variety of metals and alloys, including steel, aluminum, bronze, magnesium, zinc and tin. It can also create materials of all sizes and weights, which is why sand casting is our choice process for many of our bases.
Breaking Open the Cast
After the metal as cooled and solidified, the mold is carefully broken open to and the cast is removed. While patterns can typically be reused, the mold itself cannot. Thus, we manually recreate the mold for every new piece.
Cast Iron Finishing Process
Once the cast is extracted from the mold, it is subjecting to a finishing process. Finishing the cast iron removes the impurities from the piece and protects it from corrosion. During this process, the surface of the cast is first filed down in order to produce a smoother texture while also removing impurities in the metal.
Surface texture can be measured using RMS or Root Mean Square. RMS is the calculated root mean square average the microscopic profile peaks and valleys. A lower RMS equals a smoother surface. We typically file our iron down to be within the range of 60-300 RMS.
After filing the metal down, the piece is then either painted or sealed with a clear coat, depending on the piece. This is done in order to protect the piece, making it corrosion resistant.
Cast Iron Pros and Cons
- Perfectly Imperfect - Since every piece is casted and finished by hand, there are minor imperfections and variations between every piece. These imperfections provide the uniqueness inherent in handcrafted items, without compromising the structural integrity of the piece.
- Rustic Result - By following traditional metallurgic methodology, our bases have just the right amount of impurities that serve to contribute to the overall rustic aesthetic of the piece. Much like metalwork found in a barn or farmhouse, our cast iron bases exude a hardy, yet welcoming charm.
- Easy Maintenance - Iron is an incredibly strong material that, when finished, is corrosion resistant and thus requires minimal maintenance beyond occasional dusting.
- Lower Strength than Forged Iron - Since the iron is melted down and poured into a mold, the molecular structure changes. Because of this, cast iron is strong but stiff, giving it little flexibility. Forged iron, on the other hand, retains its original molecular structure which allows it to support more weight, since it can bend more without breaking. This isn't an issue for furniture - this difference is only noticeable when the iron is bearing a load weighing more than 3 tons. One of our cast iron tables can support the weight of a truck without an issue.
Some of our other iron bases consist of iron pieces that are first forged, then welded together to form the final product. Iron forging has been around since ancient Mesopotamia, around 6,000 years ago. Since then, many generations of blacksmiths and metallurgists have refined this process into an artform.
Forging is the process of using heat & physical pressure in order to give form a metal into a desired shape. We typically do this by hand, using a hammer and anvil - in accordance with traditional techniques - but it can also be accomplished by using specialized machinery.
Why We Use Forged Iron for Furniture
- Strong, yet Flexible - The forging process retains the iron's molecular structure, resulting in a piece that has superior strength when compared to casting.
- Industrial Aesthetic - Forged bases are simple and strong, which are core principles of industrial modern design. The sleek, geometric forms resulting from forging an iron base serve to complete the look of an industrial modern piece.
- Eco-Friendly - One of the hidden strengths of the forging process lies in its simplicity. Because iron forging is a relatively simple process when compared to other ironworking processes, there are less rejected products. This reduces the amount of scrap generated from the manufacturing process, which in turn reduces the amount of materials requiring disposal. This makes forged iron not only stronger than many other types of processed iron, but more eco-friendly as well.
How It's Made: Iron Forging Process
There are many different methods for forging iron, including hot forging, cold forging, die forging, and rolled ring forging, and more. We choose to use the hot forging process. This process is relatively simple, and involves 4 main steps: Heating, Shaping, Quenching, and Finishing.
Before pressing the metal into shape, it is typically necessary to heat the metal to the point where it is malleable but not molten.
This process is accomplished by using a forge. Forges use fuel and air in order to heat the metal to a desired temperature. It is important that the forge is able to reach the correct temperature to soften the metal, but not so hot that it melts it. For iron, the appropriate temperature is 2500° Fahrenheit, or 1371° Celsius.
Once the iron is properly heated, it can then be molded into shape. In order to do this, the metal must be first held securely in place using tongs, vises, and/or clamps, depending on the current shape of the material and the desired end result.
After the piece is secured, it is molded into shape using a hammer & anvil. The hammer and anvil are the most critical components of the forging process, as the quality of these tools determine the quality of the end result. This is why we only use the highest quality tools for this process.
There are several techniques used for shaping the iron, but there are three main ones that form the foundation:
- Drawing Out - The process of using force to draw the metal out of itself, increasing its length while also making it thinner.
- Upsetting - The process of applying force at the end of a piece in order to add volume or width
- Peening - The process involving the application of force to move metal in a specific direction. There are many different types of peens that can be used to create a variety of effects.
After a metal is shaped, it must then be cooled. To accomplish this, the metal is placed in a quenching bucket. This bucket contains a unique mixture of mineral oils that harden the iron, as well as minimizing the the chances of warping or cracking.
Forged Iron Finishing Process
Once the iron has undergone the forging process, it is then finishing via powder coating. Powder coating is a high-quality finish found on thousands of products you come in contact with each day. Powder coating protects industrial machinery as well as many household items, including furniture. It provides a more durable finish than liquid paints can offer, while still providing an attractive finish.
Powder coating consists of a 4-step process:
- Blasting - Blasting removes any dirt, grime, or corrosion. All of our forged bases are sand-blasted, as it is the best blasting process for furniture making.
- Prepping - Prepping consists of applying a metal prep chemical solution to remove surface impurities, clean off any lingering debris, and make the piece corrosion-resistant.
- Powdering - Once the piece is prepped, the coating is then applied. This is done through electrolysis, in which the metal object is positively charged in order for the powder to be attracted to it and bond well during the next stage. There are outdoor-rated and indoor-rated powders. Please note that our powder coating is rated for indoor use only. Placing our pieces outside may damage the piece's coating, making it vulnerable to corrosion.
- Curing - This is the process of heating the powder-coated piece in order to strengthen the chemical bonds between the coating and the metal. The cure times for metal can range depending on how long it takes the oven to get to temp and the size of the oven. Typical curing temperature ranges from 250°F (low cure) to 400°F are required for the powder to heat up enough to bond.
Forged Iron Pros & Cons
- Extremely Strong - Forged iron is less porous than cast iron since it is not melted prior to shaping. Because of this, the end product with superior strength.
- Simple & Sleek - Since forged iron pieces tend to have simpler shapes with sleek, straight edges, These pieces lend themselves well to modern fusion designs. The geometric shapes and simplistic nature of the piece perfectly fit modern design principles.
- Limited Sizes - The forging process is best suited for smaller pieces. Larger pieces are simply too big to fit on an anvil, which is why sand casting is the preferred method for large pieces, such as consoles or cabinets.
- Limited Shapes - More intricate and complex bases are better suited for casting, as hand-forging has limited accuracy when it comes to fine details.
How We're Making Green Iron
We believe that great furniture doesn't need to hurt the environment. World Interiors is committed to helping repair the environment and minimizing the ecological impact of the global furniture industry.
Iron is a strong, durable material that can be salvaged & recycled without sacrificing its qualities. Because of this, we elect to use only reclaimed iron. This iron comes in the form of remnants from our casting process or as scrap from various salvage yards. This ensures that we can create high quality products while also minimizing our overall carbon footprint, since we do not add to the demand for mined resources.
Both cast and forged iron are extremely durable, corrosion-resistant, and strong. They require almost no maintenance outside of periodic cleaning. Here's some tips for keeping your base clean:
- We recommend dusting your base once a week to remove contaminants.
- If something spilled onto the material, wipe it off with a rag or sponge soaked in warm soapy water. Avoid using harsher chemicals like bleach, as this will wear down the finish.
- For additional protection, you can apply a light coating of non-abrasive car wax. This will serve to further ensure that finish is protected from sun damage, however this is unnecessary for any pieces that will not suffer prolonged exposure to direct sunlight.