Word wrap in a merged cell in table

Question

I have created a simple 3-column table, in which the first row has 3 columns, and in the second row the columns are merged into one. The text in the second row is very long.

\documentclass{article}
\usepackage{tabularx}
    
\begin{document}
  \begin{center}
    \begin{tabularx}{\linewidth}{|r|X|r|}
        \hline
        \cellcolor{black!25}Techniques & \cellcolor{black!5}Artifacts & \cellcolor{black!5}Budget \\
        \hline
        \multicolumn{3}{|c|}{Island Museum analyzes historical artifacts using one or more techniques described below – all but one of which is performed by an outside laboratory – to obtain specific information about an object’s creation. For each type of material listed, the museum uses only the technique described:}
        \hline
    \end{tabularx}
  \end{center}
\end{document}

Obviously, as LaTeX does not automatically wrap text inside a table, the second row text extends past the table edge (and even page edge).

The usually suggested solution to text wrapping inside a table is to set column width (fixed or relative) when beginning the table. I tried that, e.g. \begin{tabularx}{\linewidth}{|p{0.25cm}|X|p{0.25cm}|} but that doesn't help.

Answer 1

A c type column is as wide as its widest contents. A \multicolumn{...}[c}{...} containing a lenghty text will therefore let its contents protrude into the margins and beyond the page, irrespective of the other column types used in the table.

To overcome this, we can easily calculate the required width. Since your table is supposed to be exactly as wide \textwidth, so is your \multicolumn cell. Since there are vertical lines to either side of your \multicolumn cell, let's substract their width (\arrayrulewidth) from \textwidth. There is also a small horizontal white space between the vertical line and the start/end of the text in the cell, so let's also substract that (\tabcolesp). With thouse changes you end up with the following output:

\documentclass{article}
\usepackage[table]{xcolor}
\usepackage{tabularx}
    
\begin{document}

\noindent
    \begin{tabularx}{\linewidth}{|r|X|r|}
        \hline
        \cellcolor{black!25}Techniques & \cellcolor{black!5}Artifacts & \cellcolor{black!5}Budget \\
        \hline
        \multicolumn{3}{|p{\dimexpr\linewidth-2\arrayrulewidth-2\tabcolsep}|}{Island Museum analyzes historical artifacts using one or more techniques described below – all but one of which is performed by an outside laboratory – to obtain specific information about an object’s creation. For each type of material listed, the museum uses only the technique described:} \\
        \hline
    \end{tabularx}

\end{document}

Answer 2

An answer has been added but I was not satisfied with the color scheme and the right alignment so a modification to the table used here -- \hhline not working with \multirow and \cellcolor

\documentclass{article}
\usepackage[a4paper,margin=1in]{geometry}
\usepackage[svgnames,table]{xcolor}
\usepackage{makecell, multirow, tabularx}
    \newcolumntype{L}{>{\raggedright\arraybackslash}X}
    \renewcommand\theadfont{\normalsize\bfseries\color{white}}
\usepackage{hhline}

\definecolor{headers}{RGB}{0,137,182}
\definecolor{M1}{RGB}{163,195,217}
\definecolor{M2}{RGB}{155,214,220}


\begin{document}
    \setlength\arrayrulewidth{2pt}
    \arrayrulecolor{white}
    \def\clinecolor{\hhline{|>{\arrayrulecolor{M2}}->{\arrayrulecolor{white}}|-|-|}}
    \begin{tabularx}{\textwidth}%
    {|>{\hsize=0.5\hsize\columncolor{M2}}L|
      >{\columncolor{M2}}L|
      >{\hsize=0.5\hsize\columncolor{M2}}L|
    }
    \rowcolor{headers}
    \thead{Techniques }          & \thead{Artifacts}                & \thead{Budget}         \\
    \hhline{|-|-|-|}
    \rowcolor{M1}
            \multicolumn{3}{|p{\dimexpr\linewidth-2\tabcolsep-2\arrayrulewidth}|}{Island Museum analyzes historical artifacts using one or more techniques described below – all but one of which is performed by an outside laboratory – to obtain specific information about an object’s creation. For each type of material listed, the museum uses only the technique described:}                            \\
    \end{tabularx}
\end{document}